Add placeholder text for send testing email button in admin/config

[bagasme]

As title.

Input box for Send testing email in admin/config doesn't have any text guidance for correct usage. This PR add such in form of placeholder attribute.

Screenshot:

[techknowlogick]

Thanks for PR. For accessibility placeholders aren’t supposed to be used for instructions, but rather labels associated with input fields are (screen readers may not read out the placeholder)

As well could You update this to make the string able to be translated to other languages?

[silverwind]

I prefer example values, e.g. user@example.com. I don't think we need to localize that.

MDN says:

The placeholder attribute is a string that provides a brief hint to the user as to what kind of information is expected in the field. It should be a word or short phrase that demonstrates the expected type of data, rather than an explanatory message.

dismissing my review

[codecov-commenter]

Codecov Report

Merging #12452 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12452      +/-   ##
==========================================
- Coverage   43.81%   43.80%   -0.01%     
==========================================
  Files         630      630              
  Lines       69821    69821              
==========================================
- Hits        30589    30587       -2     
- Misses      34297    34299       +2     
  Partials     4935     4935              

Impacted Files	Coverage Δ
modules/queue/workerpool.go	58.77% <0.00%> (-1.23%)	⬇️
modules/git/repo.go	51.26% <0.00%> (-1.02%)	⬇️
modules/log/event.go	56.60% <0.00%> (-0.95%)	⬇️
modules/notification/webhook/webhook.go	48.43% <0.00%> (-0.53%)	⬇️
models/issue.go	56.78% <0.00%> (-0.36%)	⬇️
services/pull/pull.go	43.41% <0.00%> (-0.24%)	⬇️
models/repo.go	55.71% <0.00%> (+0.15%)	⬆️
modules/indexer/stats/db.go	60.86% <0.00%> (+17.39%)	⬆️
modules/indexer/stats/queue.go	76.47% <0.00%> (+23.52%)	⬆️
modules/util/timer.go	85.71% <0.00%> (+42.85%)	⬆️

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