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
Merge pull request #6524 from brettp/site_secret_18

chore(admin): Moved site secret regeneration to advanced settings.
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
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.