Split picture and pictureset templates. Add 2 new settings.

a) inline_media/inline_media_picture.html has been split into 3
templates:

  * inline_media/inline_media.picture.mini.html:
     Used only for inline_mini_* css classes.
  * inline_media/inline_media.picture.default.html:
     Used for inline_small_*, inline_medium_* and
     inline_large_* css classes.
  * inline_media/inline_media.picture.full.html:
     Used only for inline_full_* css classes.

  The same for picturesets.

b) Two new settings:

  1. INLINE_MEDIA_TEXTAREA_ATTRS, see settings.rst in the docs.
  2. INLINE_MEDIA_CUSTOM_SIZES, see settings.rst in the docs.
Merge branch 'feature/customsizes'