fix(groups): forces content accessibility to members_only for invisible groups

When user selects "Group members only" for group visibility, javascript sets
the group content accessibility field to "Members only" and disables it.

For existing groups the above is done also on server side.

When editing an existing public group, a warning is added under the group
content accessibility input saying that changing the setting won't change
the access permission of existing group content.
feature(externalpages): page layout changed to one_column

fixes #6535
Merge pull request #6480 from hypeJunction/pages-fixes

fix(pages): use ElggBatch to avoid memory leaks
Merge pull request #6537 from jdalsem/externalpages-layout

feature(externalpages): page layout changed to one_column
fix(hooks): return values defaults to null in javascript

Fixes #6499
fix(hooks): handlers returning null/undefined don't change returnvalue in javascript

Fixes #6531
Merge pull request #6532 from sembrestels/js-hooks-fixes

Normalizes the JavaScript elgg.trigger_hook to behave like PHP's
feature(uservalidationbyemail): forwarding to an info page after registration

Replaced system message by an informative page.
Removed system message when users try to log in, there is already another
message explaining why they can't log in.

Fixes #6247
fix(embed): file embedding wasn't working for textareas

Issue was introduced in 8b358c5 by removing the line that applies the text
change in textarea.

The way to extend javascript from the rich text editor was to extend a view
in js/embed/embed. It has been refactored to the 'embed', 'editor' js hook,
in order to make js/embed/embed convertible to js file. The view extension
has been deprecated.

Text areas and rich text editors manage the insertion diferently. In order
to insert the content in the correct place, we allow rich text editors to
return false inside the hook to stop the insert, and apply it just once.

Fixes #6160
chore(uservalidationbyemail): cleans up implementation

This moves the page handling logic to separate scripts and also
replaces direct $_SESSION access with the elgg_get_session() API.
Merge pull request #6514 from sembrestels/uservalidationbyemail-infopage

User validation by email info page
fix(admin): fixed javascript error on toggle simple cache checkbox

Fixes #6529
Merge pull request #6550 from sembrestels/admin-js-fix

fix(admin): fixed javascript error on toggle simple cache checkbox
fix(css): reposition powered by elgg

Fixes  #6534
Merge pull request #6549 from mrclay/sembrestels-uservalidationbyemail-infopage

chore(uservalidationbyemail): cleans up implementation
Merge pull request #6551 from PerJensen/powered_by_elgg

fix(css): reposition powered by elgg
Merge 1.8 into master.
chore(ui): Improved site secret UI for master after 1.8 merge.

Refs #6443
chore(git): Setting the msg validation script executable so symlinking for commit-msg hooks works.
chore(travis): Ignoring all merge commits.

Refs #5976