[ticket/11628] Remove style resource locator

No longer used since Twig was implemented.

PHPBB3-11628
[ticket/11628] Remove style path provider

No longer used since Twig was implemented.

PHPBB3-11628
[ticket/11628] Remove phpbb_style (move methods to phpbb_template)

PHPBB3-11628
[ticket/11628] Remove $this->names, $this->style_names from phpbb_template

These are not used anywhere

PHPBB3-11628
[ticket/11628] Move setting core namespace to set_style()

PHPBB3-11628
[ticket/11628] Return $this from set_style, set_custom_style

PHPBB3-11628
[ticket/11628] Remove third parameter ($names) from set_custom_style

This was basically duplicating functionality. $names would be used if not
empty, else array($name) would be used. Merged functionality into the first
argument

PHPBB3-11628
[ticket/11628] Change set_custom_style $template path to default to string

Rather than default to false and compare === false ? 'template/' : value just
assign this default in the arguments

PHPBB3-11628
[ticket/11628] Set admin namespace in the constructor

PHPBB3-11628
[ticket/11628] Remove set_style_names function, moved to set_custom_style

PHPBB3-11628
[ticket/11628] Remove template_path option on set_custom_style

This was set to default 'template/' to append template/ to all the paths,
but every location was actually just setting it to '' to not append anything.
So removed the option entirely (additional paths can be appended to the paths
being sent to the function already)

PHPBB3-11628
[ticket/11628] Shorten an if to an inline statement

PHPBB3-11628
[ticket/11628] Remove the one usage of get_style_path()

Makes the code easier to follow

PHPBB3-11628
[ticket/11628] Fix a bug I noticed in template->destroy

Should not be setting $this->context = array()!

PHPBB3-11628
[ticket/11628] Make get_template_vars protected

Remove all references to it and the hacky code in messenger that was using it

PHPBB3-11628
[ticket/11628] Remove remaining style_path_provider_test

PHPBB3-11628
[ticket/11628] phpbb_template, not phpbb_template_interface

PHPBB3-11628
[ticket/11724] Support "ELSE IF" and "ELSEIF" in the same way

PHPBB3-11724
[ticket/11724] Replace spaces with tabs

PHPBB3-11724
[ticket/11792] Add variable 'lang_set_ext' to event core.user_setup

To allow extensions to add global language strings just like mods can, add the
'lang_set_ext' variable to the core.user_setup event. It requires an ext_name
to be specified as well as a lang_set, and loads the specified lang_set in the
context of the extension.

PHPBB3-11792