Merge latest changes into production

[tcouch]

closes #326 closes #317

Search within specific fragment content fields

Adds the ability to search within original text content, translations and commentaries separately, rather than all at once. To achieve this, new search methods have been added, and the content type drop-down list on the search page now splits fragments, testimonia, anonymous fragments, and apposita into groups. See https://www.ucl.ac.uk/history/frrant-preprod/search/

Allow users to order anonymous fragments within topics

The following changes have been made to the anonymous fragment list page:

• Pagination has been removed
• Instead, select a topic from the drop-down menu to view anonymous fragments related to that topic
• Use up/down buttons, or drag fragments to reorder them within the topic
• Overall anonymous fragment order (and hence their name) is now determined first by topic order, and then by order within topic.

Example: Anonymous fragments A and B both belong to the Monarchy topic and the Law topic. Monarchy is ordered first, followed by Law. Under Monarchy A is ordered first, and then B. Under Law B is ordered first, and then A. In this situation fragment A would be named "Anonymous F1" and B would be named "Anonymous F2" because their respective order under the first topic takes precedence over their order in the second.

[tcouch]

Merging this into production now since everyone had a look yesterday and no issues were raised.