Handle source: metatag when editing posts

[evazion]

Currently every metatag except source: is supported when editing a post. This is needed for adding sources via tag scripting as described here: http://danbooru.donmai.us/forum_topics/9127?page=131#forum_post_118649.

(Note to self: apply fixes here and here)

[Type-kun]

It wouldn't handle spaces properly, though. It's possible to make special exception in Tag.scan_tags, for example extracting source:"text with spaces here" first before breaking the rest on whitespace, but then it wouldn't handle double quotes.

[evazion]

source: doesn't handle spaces properly when searching either. So we already can't handle things like source:Image board or source:*Kantai Collection*. The best you can do is abuse wildcards: source:Image*board, source:*Kantai*Collection*.

Also there's no way to search for a literal source of none or literal * characters. Such posts do exist, although only a small number.

I think the options are:

• Fix Tags.parse_query to allow source:"*Kantai Collection*". But that doesn't look easy.
• URL decode the source string. So to search *Kantai Collection* you'd say source:*Kantai%20Collection*. But that's not easy to use.
• Only allow URLs in the source. If the source is a doujin or something then put it in a pool instead of putting the title in the source.
• Don't worry about all this, just leave it as is.

[Type-kun]

Took a stab at the hardest version. Searching for source:"*Kantai Collection*" results in the following where condition:

(SourcePattern(lower(posts.source)) 
LIKE SourcePattern(lower('%Kantai Collection%')) ESCAPE E'\\')

That is, spaces are handled correctly, and asterisks are allowed within double quotes too. Seems pretty decent unless it broke something else. It doesn't fix searching for literal none or * - I think it's possible to fix, too, but it requires changes in other places.

[Type-kun]

Fixed with the merged PR