Cache Banzai projects/objects using RequestStore

This was originally suggested by @ayufan and modified to be a bit
cleaner and use RequestStore instead of a regular Hash.

By caching the output of the two methods involved the number of queries
is reduced significantly. For example, for an issue with 200 notes (of
which 100 reference a number of merge requests) this cuts down the
amount of queries from around 6300 to around 3300.