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
Full rewrite 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The old move_db2disk.php program was completely outdated and did not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

Conflicts on cherry-pick from master-1.2.x:
	admin/move_db2disk.php
	admin/system_utils.php
Fix redirection to system utils page after moving attachments