Limit breadcrumb's message length

[st0012]

Sentry::Event's message has a 8096 size limit, which should also be applied to breadcrumb's message to prevent useless and even harmful data.

This should partially fix #1422

[codecov-commenter]

Codecov Report

Merging #1465 (41dce08) into master (a72ddc7) will increase coverage by 0.59%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1465      +/-   ##
==========================================
+ Coverage   98.24%   98.84%   +0.59%     
==========================================
  Files         214      119      -95     
  Lines       10142     6316    -3826     
==========================================
- Hits         9964     6243    -3721     
+ Misses        178       73     -105     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/breadcrumb.rb	100.00% <100.00%> (ø)
sentry-ruby/spec/sentry/breadcrumb_spec.rb	100.00% <100.00%> (ø)
sentry-rails/app/jobs/sentry/send_event_job.rb	72.22% <0.00%> (-5.56%)	⬇️
sentry-rails/lib/sentry/rails/active_job.rb	97.05% <0.00%> (-0.09%)	⬇️
sentry-rails/lib/sentry/rails/configuration.rb	100.00% <0.00%> (ø)
...-sidekiq/spec/sentry/sidekiq/error_handler_spec.rb	100.00% <0.00%> (ø)
sentry-raven/lib/raven/transports/dummy.rb
sentry-raven/lib/raven/interfaces/message.rb
...y-raven/spec/raven/processors/http_headers_spec.rb
sentry-raven/spec/raven/json_spec.rb
... and 91 more

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