Create admin-overridable constants

A fair number of requests are for tweaking various constants in Metabase. We've picked sane defaults that work well for the majority of our users, but some use-cases might dictate different values.

We could create a file that defines all the constants an admin might want to change, and a way for admins to override them. These are things that probably don't deserve a setting in the UI because the defaults are fine for ~95% of installations.

Some possible

* max rows un-aggregated / aggregated #8322 #8622
* max rows CSV/JSON download #3810 #8625
* pulse export limit #7622
* dashboard refresh periods #4299
* dashboard max cards #5827
* binning/grouping granulaties #13719 #12847 #9208
* max number of series that can be graphed #3656 #7958
* timeouts #12003
* max field values to sync / mark a column as a category
* row limits in the UI #4544
* connection limits #2172

Feel free to suggest others.

### Notes from 05/14/2020 meeting

- To make these configurable we can use env vars for backend-only constants. For frontend ones we can define `:public` Settings which are still configurable via env var but automatically passed to the frontend.
- As part of this issue we should be sure to document all of the new env vars.

⬇️ **Please click the 👍 reaction instead of leaving a `+1` or 👍 comment**

Start by reviewing the constants listed in the issue and the 05/14/2020 meeting notes, then trace where those limits are defined and consumed in the backend and frontend. Determine which values should use backend environment variables versus public Settings, and identify the documentation needed for each new environment variable. Done means the scope and configuration approach are agreed and the resulting variables are documented.

Written by the indexing model from the issue text.