Change Thesis Project / Theme description from Text to MediumText

It seems the 65.000 character limit is a problem given the types of content that some users attempt to embed (e.g. images).

As an initial fix, we should increase the limit by changing the description field from Text (64 KB) to MediumText (16 MB). If we overhaul how images work in relation to the markdown editors, we can reconsider this change again.