Fix #15558: url_get() fallback to next method in case of error
Localisation updates from http://translatewiki.net.
Fix invalid access denied error caused by access_has_bug_level()

Commit d42e80c01277eec37c7436a23ec72073c9961c49 changed the code in
access_has_bug_level() function, and introduced a bug in the call to
access_compare_level() when access is limited to issue reporter
(inverted parameters caused an incorrect access denied error).

See also 9ec47a056156ac32e1f95cd0ec20d4db8bd54d3b.

Issue #15538
Print pages: don't show custom fields user has no access to

Prior to this, if a custom field's 'read' access level were higher than
the user's access, the print pages would display the field's label (but
not the data). This behavior is not consistent with how this situation
is handled in the View Issue page, where the field is not shown at all.

For consistency, we now skip the of the custom field users have no read
access to in the print pages, so that nothing is displayed.

Fixes #15528
Removed unused API function project_file_is_name_unique()
Modify file_is_name_unique() API to work for project files also

Previously this function would only work for bug attachments, and
another API function project_file_is_name_unique() would take care of
projects. The latter was removed as it was not used, and this one
amended in case it's ever required to check for uniqueness of a project
file.

It also makes sense to regroup file-related functions in file_api.php.

Issue #15572
Revise and improve diskfile_is_name_unique()

Prior to this, the API would only check for a file's uniqueness by
looking up the bug_file table, and did not actually checking the file
system for an existing file.

This causes 2 potential issues:

- we could have a duplicate in the file system and therefore unwittingly
  overwrite it with a new file
- it was not possible to use the function for for project docs

Fixes #15572
PHPdoc updates
Refactoring the Move Attachments admin page

The old move_db2disk.php program was completely outdated and did not
work properly.

This commit introduces completely new page, with dissociated page to
select which attachments to move, and action page to actually move the
files from DB to disk

Fixes #15496