Snippet repository size cache expiration

(for backlog grooming people, this is done!)

Summary

Currently snippet repository size is cached, but is not expired when new content is pushed to the server.

Steps to reproduce

1. Create snippet
2. Open a rails console, find that snippet
3. Get its size by snippet.repository.size
4. Push new content onto snippet
5. Get its size by snippet.repository.size

What is the current bug behavior?

Snippet size returned does not change.

What is the expected correct behavior?

Snippet size returned changes.

Output of checks

12.9

Possible fixes

Expire the cache during the code path triggered by file pushes.