Fix low-hanging fruit in English (source) translation

[bugreport0]

This PR fixes the low-hanging fruit from issue #3512 which is non-subjective:

• ellipses are used instead of repeated periods
• spacing before ellipses is now consistent (no space)
• one missing 'a' is added
• 'an' is used before words starting with a vowel
• the term 'username' is consistently used

[codecov-io]

Codecov Report

Merging #3518 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3518      +/-   ##
==========================================
+ Coverage   35.68%   35.69%   +<.01%     
==========================================
  Files         281      281              
  Lines       40701    40701              
==========================================
+ Hits        14524    14527       +3     
+ Misses      24029    24028       -1     
+ Partials     2148     2146       -2

Impacted Files	Coverage Δ
models/repo_indexer.go	48.3% <0%> (ø)	⬆️
modules/process/manager.go	81.15% <0%> (+4.34%)	⬆️

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[lunny]

LGTM

[bugreport0]

Hurray for the LGTM, but maybe we should aim for structural improvements here. Would it be possible to / can someone write a unit test that greps all translations to make sure that:

• no occurrences of three repeated periods '...' are allowed
• no occurrences of an ellipsis with a space in front ' …' are allowed

This would document and codify some initial translation rules. We could possibly expand on that in the future.

[lunny]

@bugreport0 That unit test could be another PR. Let's merge this one at first.