[feature/soft-delete] Lay the groundwork for a soft-delete feature

So far, I've added no new functionality.  The biggest change here is adjusting the DB column names to "visibility" rather than "approved".  Some things here are pretty likely to change, for example the name and location of the topic_visibility class. Happy birthday phpBB :)

PHPBB3-9657
[feature/soft-delete] Correct some mistakes in e8d47

Notably: Uncomment the die() in create_schema_files, and add the class that makes everything tick.

PHPBB3-9657
[feature/soft-delete] I told you I was going to rename the class!

Rename topic_visibility class to phpbb_visibility. Also a bit of work to the class itself, mostly cleanup and adding the comments that I'd previously written.

PHPBB3-9657
[feature/soft-delete] Implement the ability to soft-delete and restore posts

The soft delete feature seems to work.  Tests are pending.  A real icon is pending.  Add the permissions and the interface to soft-delete posts.  Also able to restore posts via the MCP queue

PHPBB3-9657
[feature/soft-delete] Add unit tests for the phpbb_visibility class

Add unit tests for the phpbb_visibility class.  Adjust the phpbb_visibility class to pass those unit tests.  The changes are pretty small, actually.

PHPBB3-9657
[feature/soft-delete] Rename phpbb_visibility class to phpbb_content_visibility

Rename the class to more accurately reflect what it does.

PHPBB3-9657
[feature/soft-delete] Add a processor for action == restore in mcp_queue.php

Restoring a post within mcp_queue.php didn't do anything before this commit.  Now it does, by way of a function which is very similar to approve_post.

PHPBB3-9657
[feature/soft-delete] Fix some small bugs

To wit: using non-existing constant POST_DELETED in posting.php; first test
post was initially unapproved; soft delete checkbox appeared at post time
Links pointing to the wrong place.

PHPBB3-9657
[feature/soft-delete] Use autoloading for content_visibility class
PHPBB3-9657
[feature/soft-delete] Fix typo in column name topic_visibility

PHPBB3-9657
[feature/soft-delete] Fix the schema files to the correct line ends again

PHPBB3-9657
[feature/soft-delete] Remove imageset/ which was created by merge conflict

We don't have imagesets in 3.1 anymore.

PHPBB3-9657
[feature/soft-delete] Fix displaying of "deleted post" note in viewtopic

PHPBB3-9657
[feature/soft-delete] Fix missing AND in query

PHPBB3-9657
[feature/soft-delete] Use the variable which holds the correct item status

PHPBB3-9657
[feature/soft-delete] Update restoring feature to use ajax if requested.

Also fixes the mcp as a hole:
- displayes a success message
- gives a link to the post, if only one was restored

PHPBB3-9657
[feature/soft-delete] Update posts and topics table with database_update.php

PHPBB3-9657
[feature/soft-delete] Simplify the query output if the user has m_restore

PHPBB3-9657
[feature/soft-delete] Simplification part2: user can see all item visibilities

If the user can see all visibilities, we can simply leave out the query part,
instead of adding a bunch of ANDs.

PHPBB3-9657
[feature/soft-delete] Fix copy-paste fail for database update on the index

PHPBB3-9657