[ticket/11538] Make sure group color can't exceed maximum of 6 characters

In order to prevent future issues with this, a basic set of functional
tests for the UCP groups manage page have been added.

PHPBB3-11538
[ticket/11538] Use regex for testing color value and improve tests

We are now using a regex with preg_match() in order to properly check
if the entered color value is in hex color format or not. A proper
error message is triggered if an incorrect color value is entered and
the prepended '#' is removed if necessary.

PHPBB3-11538
[ticket/11538] Fix incorrect regex and test for duplicate # in color string

PHPBB3-11538
[ticket/11538] Simplify colour value check and remove support for '#'

The input length for the hex color is now limited to 6 characters and
the support for colors starting with a '#' has been dropped. The allowed
input length of 7 in prosilver seems to have been a relict from old ages
of phpBB3. In order to have proper support for correct checking of the
colour value, the new code was also ported to the ACP groups manage page.
The tests have been modified to reflect the changes to the behavior of
the color check. Tests for the ACP will follow.

PHPBB3-11538
[ticket/11538] Add tests for acp group manage page

This will currently test if the colour check properly works.

PHPBB3-11538
[ticket/11538] Make sure regex doesn't allow multiple color values

This will now make sure that 'AAFF00' will be matched but strings like
'AAFF00 AABB00' will not.

PHPBB3-11538
[ticket/11538] Add function phpbb_validate_colour for validating colours

It will be possible to use this function via the validate_data() function
interface that has already been used previously. Thus, this new function
will extend the capabilities of validate_data() to checking hex color
values.

PHPBB3-11538
Merge branch 'develop-olympus' of https://github.com/phpbb/phpbb3 into ticket/11538
[ticket/11538] Use abstract class for functional test cases for group colour

PHPBB3-11538
[ticket/11538] Rename phpbb_validate_colour to phpbb_validate_hex_colour

PHPBB3-11538
[ticket/11538] Merge calls to validate_data() in acp_groups

PHPBB3-11538
[ticket/11538] Move group ID into abstract test class and add more test cases

The group ID is defined in the abstract class now and additional test cases
for hex colour values have been added.

PHPBB3-11538
[ticket/11538] Limit comment in acp_groups to 80 characters per line

PHPBB3-11538
[ticket/11538] Modify test colour values

PHPBB3-11538
[ticket/11538] Add optional switch as argument to hex colour validation

The value of $optional will decide whether an empty string will be
treated as incorrect input or if it is allowed. The optional argument
will default to false and therefore treat an empty string as incorrect
unless explicitly told to not do so.

PHPBB3-11538
Merge remote-tracking branch 'marc1706/ticket/11538' into develop-olympus

* marc1706/ticket/11538:
  [ticket/11538] Add optional switch as argument to hex colour validation
  [ticket/11538] Modify test colour values
  [ticket/11538] Limit comment in acp_groups to 80 characters per line
  [ticket/11538] Move group ID into abstract test class and add more test cases
  [ticket/11538] Merge calls to validate_data() in acp_groups
  [ticket/11538] Rename phpbb_validate_colour to phpbb_validate_hex_colour
  [ticket/11538] Use abstract class for functional test cases for group colour
  [ticket/11538] Add function phpbb_validate_colour for validating colours
  [ticket/11538] Make sure regex doesn't allow multiple color values
  [ticket/11538] Add tests for acp group manage page
  [ticket/11538] Simplify colour value check and remove support for '#'
  [ticket/11538] Fix incorrect regex and test for duplicate # in color string
  [ticket/11538] Use regex for testing color value and improve tests
  [ticket/11538] Make sure group color can't exceed maximum of 6 characters