Revert "Issue #12545: Enforce XHTML strict output for all pages served"

This reverts commit 4fc1d8b8290872e660c6d9a179c4c2209adf7e26.

Fixes #13304, #14759, #15446

Conflicts:

	core/http_api.php
Fix #16025: Change DOCTYPE to HTML5

As discussed in mailing list [1]

[1] http://article.gmane.org/gmane.comp.bug-tracking.mantis.devel/4468
Add new helper API function check_disabled()

Prerequisite to fix issue #16024
Disable selection of projects in which user can't report issues

When the current project is 'All Projects' and user clicks on 'Report
Issue', login_select_proj_page.php presents them with a list of projects,
which includes those in which the user is not allowed to report issues.
If one of these projects is selected, an 'Access Denied' error occurs.

This commit makes the functionality more user-friendly by disabling
these projects in the list, so users can't select them.

To implement this, a new optional parameter was added to functions
print_project_option_list() and print_subproject_option_list().

Fixes #16024

Conflicts:
	core/print_api.php
Can't copy columns to/from subproject when parent is current

In the manage columns page, it is not possible to copy the columns to or
from a subproject, when the current project is the parent project,
because the selection list completely removes the whole subtree instead
of just the current project.

In addition, we now also don't print 'All Projects' anymore when the
current project is the same.

Fixes #16029

Conflicts:
	core/print_api.php
Preparation for merge of 'dregad/13x' branch

This merges a few cherry-picked commits with the appropriate conflict
resolutions, to facilitate the merge of the whole branch later on.

After this, the only remaining conflicts are the issues with the
language files and api, which will be resolved after feedback from and
discussion with grangeway.