Replace mime-types gem with mini_mime

[bradleypriest]

Replaces the MimeTypes gem with MiniMime.

Similar to mikel/mail#1059 and teamcapybara/capybara#1884, this switches out the Mime Types gem with MiniMime to save memory.

EDIT:
There seems to be some requirements around the integration between carrierwave and fog going on here.

I see a couple of options:

1. We more explicitly call out that if they're using fog, they'll need to include the mime-types gem.
2. We try and convince fog to move across to mini_mime as well.
3. We add a mime-helper module that defines the three methods we use the mime gems for which can delegate to whichever library is included in the Gemfile.
4. We don't support mini_mime

With the incredibly popular mail and capybara gems moving to mini_mime, it'd be nice to switch carrierwave across too (or at least provide the option)

[bradleypriest]

Any feedback from the team here?

[bradleypriest]

@mshibuya any thoughts either way on this change?

[bradleypriest]

This PR has been open a year now, I appreciate it's set up for the 2.0 release of the gem, which is an absolutely herculean task.

However for my own mental wellbeing (I can't deal with a PR being open this long), I'm going to close it now.

Anyone is welcome to this code at any time, whether it's by reopening this PR, or recreating a new one.

Thanks for the hard work.

[mshibuya]

Sorry for being late, we appreciate your work!