Corpus form: disable controls on active corpus

[lukavdplas]

If a corpus is active, some controls in the corpus form should be disabled, because they may affect the data format. If the user wants to change these, they must deactivate the corpus first.

This concerns:

• The corpus title (see also Corpus form: updating corpus title orphans index #1824)
• If Corpus form: filter field language options from corpus metadata #1816 is implemented, the corpus languages
• The data file
• On the field form:
  • Data type
  • Search
  • Filter (at least for text fields)
  • Languages
  • Removing / adding fields