Extend dynamic attributes to all entities

We have dynamic attributes for works, but never got as far as extending them to other entities.

Possible ideas for things which could use dynamic attributes on releases (I'm sure I could find plenty of ideas for other entities too):

• Number of copies (number) - For limited editions
• Mono/stereo (fixed list) - Generally useful for older releases which came in two editions, a mono and a stereo version. Discogs has this in their formats list (http://www.discogs.com/help/submission-guidelines-release-format.html)
• UK single number (fixed list: CD1, CD2, rarely also CD3) - UK singles used to regularly come in multiple editions but there's no easy way to getting this info for things like tagging (you have to try and parse it out of the disambiguation).
• JASRAC license numbers (text: 7 digit number prefixed with "R-", "T-" or "V-", possibly more)
• SPARS code (https://en.wikipedia.org/wiki/SPARS_code) (fixed list)
• Code 39 barcodes (https://en.wikipedia.org/wiki/Code_39) (text) - This would give us a way to implement MBS-3850 without having to change the current behaviour of the barcode field, which would be good, because these are very obscure compared to normal barcodes. We could even go as far as defining the existing field as only EANs/UPCs (related: MBS-7419).
• Distribution codes (http://wiki.discogs.com/index.php/Distribution_Codes) (text)
• Price (as printed on the release) (text?) - Japanese releases have the minimum retail price printed on them (http://nippop.com/features/Saihan_Seido___Japan_s_Resale_Price_Maintenance_System/). It's not so common for music releases outside of Japan, but things like books and magazines (which sometimes have free CDs) would most likely have a price printed on them. If we were to add this, we would need to consider how it interacts with digital releases. VGMdb stores prices.

There are also some things supported by Discogs (http://www.discogs.com/help/submission-guidelines-release-barcode.html) which sometimes get pasted into annotations because we don't have any proper fields to store them:

• Mastering SID code (text)
• Mould SID code (text)
• Matrix / Runout (text)
• Label Code (text) - We can store this on labels, but that doesn't tell us what is on the release itself. There have been cases of label codes changing, if there are multiple labels, we don't know which ones' label code is on the release, nor do we even know if there is a label code printed on the release.

Other requests which could probably be implemented the same way:

• MBS-4840
• MBS-7360
• MBS-7418
• MBS-7449
• MBS-8309