Re-position CaptureExceptions middleware to reduce tracing noise

[st0012]

Right now, sentry-rails is smart enough to un-sample transactions of static file/assets requests.

But users may still see tracing logs in those requests like:

[Tracing] Starting <rails.request> transaction </favicon.ico>

This is because the current implementation un-samples those transactions after the initial sampling decision has been made.
The approach's downsides are:

1. It requires hacking the FileHandler class.
2. It generates annoying and sometimes confusing tracing logs, like the above one, for requests that are always un-sampled.

The reason behind it is the order of middlewares:

request 
  -> Sentry::CaptureExceptions (starts the transaction) 
  -> ActionDispatch::Static (checks if the request is for static files) 
      -> A patch from the SDK then manually un-sample the transaction if the request is for static files

As you can see, when starting the transaction, the SDK can't possibly know if the request is for static files unless Sentry::CaptureExceptions also knows how to check static file requests.

Solution - Re-position the middleware

When CaptureExceptions is the first middleware, the benefits are:

1. It captures as many exceptions as possible.
2. Transactions can record almost the full length of the request's events.

But there are also some downsides:

1. Exceptions of the outter middlewares may mostly be noise to users, as they usually are the Rails default.
2. It makes skipping some requests' transactions very hard. For example: Un-sample asset/static file requests in a less-hacky way #1404

By re-positioning the middleware after ActionDispatch::Executor, we can avoid the downsides while still keeping most of the benefits. It also allows us to remove the patch on FileHandler.

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@f82a2c3). Click here to learn what that means.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master    #1405   +/-   ##
=========================================
  Coverage          ?   98.47%           
=========================================
  Files             ?       33           
  Lines             ?     1113           
  Branches          ?        0           
=========================================
  Hits              ?     1096           
  Misses            ?       17           
  Partials          ?        0           

Impacted Files	Coverage Δ
...entry-rails/lib/sentry/rails/capture_exceptions.rb	96.00% <100.00%> (ø)
sentry-rails/lib/sentry/rails/railtie.rb	100.00% <100.00%> (ø)
sentry-rails/spec/sentry/rails/tracing_spec.rb	99.13% <100.00%> (ø)
sentry-rails/spec/sentry/rails_spec.rb	100.00% <100.00%> (ø)

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