Add dev routes to directly test rendering of any mail or template

[jinroh]

This PR adds the following routes in a dev release:

• GET /dev/mails/:name
• GET /dev/templates/:name

These routes can be used to help the development and directly render a specific mail or template. The query parameters given are used a input data for the template rendered. For the mail route, it possible to specify a ContentType query parameter to render text/plain or text/html mail.

[codecov-io]

Codecov Report

Merging #1145 into master will decrease coverage by 0.06%.
The diff coverage is 7.31%.

@@            Coverage Diff             @@
##           master    #1145      +/-   ##
==========================================
- Coverage   85.11%   85.04%   -0.07%     
==========================================
  Files         176      177       +1     
  Lines       52163    52204      +41     
==========================================
+ Hits        44396    44399       +3     
- Misses       6341     6379      +38     
  Partials     1426     1426

Impacted Files	Coverage Δ
web/dev.go	0% <0%> (ø)
pkg/workers/mails/mail_templates.go	93.28% <0%> (-0.3%)	⬇️
web/statik/handler.go	53.29% <0%> (ø)	⬆️
web/routing.go	82.05% <100%> (+0.47%)	⬆️
web/middlewares/basic_auth.go	0% <0%> (ø)	⬆️

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