Add rack-mini-profiler gem

[coalest]

Description

We have some pages that are known to be slow. Adding this gem might make slow performance somewhat more obvious for contributors and potentially easier to debug.

For example, in one issue I remember a contributor asking for a flamegraph to triage a slow request. Adding this gem lowers the bar for contributors to work on performance-related issues.

I also thing it might help when testing PRs or features, to see if a PR is adding a N+1 or something that will cause a performance degradation.

Documentation on the gem can be found here: https://github.com/MiniProfiler/rack-mini-profiler

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Type of change

• Internal (gem only added to development env)

How Has This Been Tested?

I don't think testing is necessary

Screenshots

Some example screens:

[dorner]

@coalest do we still need bullet if we have this?

[coalest]

I still find it useful for the following reasons:

1.It provides a warning when eager loading an association that then isn't used/needed (with rack mini profiler, you would have to go through the SQL queries and check manually for the extra query).
2. And it's still helpful for catching N+1's because with the seed data, we may only have a few records on an index page. So an N+1 might cause 3 queries instead of 1, which isn't very noticeable just by looking at the SQL query count.