Compare similarity graph breaks with several terms at once

The 'compare similarity' graph seems to crash when requesting several terms at the same time.

Notes:
- This does not happen locally
- Each term makes a separate HTTP request so there is nothing special about the requests themselves.

I'm guessing this is because requesting several terms at once causes time-outs. This could be resolved by running these as celery tasks.

Start by locating the compare-similarity graph entry point and the code that issues one HTTP request per term. Reproduce the failure with several terms, then investigate whether request time-outs cause the crash and review how Celery tasks are used in the project. Done means multi-term comparisons no longer crash and the behavior is covered by an appropriate test.

Written by the indexing model from the issue text.