Deprecate rack_profiler:install in favour of rack_mini_profiler:install

[gmcgibbon]

The gem's name and install generator should be the same.

This deprecates the current generator in favour of a more appropriately named one. It also updates copy on the initializer comments and usage clarifying what it is doing and when to use an initializer. Behaviour is changed for the install generator to always generate a rack_mini_profiler.rb initializer instead, but I think that's acceptable if we want to rename the generator.

[codecov-commenter]

Codecov Report

Merging #550 (dd4e83b) into master (edbcad3) will not change coverage.
The diff coverage is n/a.

❗ Current head dd4e83b differs from pull request most recent head 8cea50f. Consider uploading reports for the commit 8cea50f to get more accurate results

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@@           Coverage Diff           @@
##           master     #550   +/-   ##
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  Coverage   87.39%   87.39%           
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  Files          18       18           
  Lines        1269     1269           
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  Hits         1109     1109           
  Misses        160      160           

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[SamSaffron]

I think the reason I was holding off on merging stuff was cause our test suite is not passing, looks unrelated though.