Rename ctx.Form() to ctx.FormString() and move code into own file

[6543]

Followup from #16562 prepare for #16567

• Rename ctx.Form() to ctx.FormString()
• Reimplement FormX func to need less code and cpu cycles
• Move code into own file

[zeripath]

I don't understand the reasoning behind renaming form to formstring.

It appears to make it inconsistent with the way we use Query and with ini.

Is the intention that form will return the form in the future?

[6543]

@zeripath the problem is that ctx.Req.Form != ctx.Form at the moment ... a name is colliding :/
-> this is more of a code readability problem - the code will just work fine without rename

to the form thing ... it's golang specific - i personaly also prevere to use the term query ... but net/http do differ between GET and POST, ... requests - Query only apply to get where Form do the same and apply to all request types

[codecov-commenter]

Codecov Report

Merging #16571 (03b6dea) into main (d9ef43a) will increase coverage by 0.01%.
The diff coverage is 53.09%.

@@            Coverage Diff             @@
##             main   #16571      +/-   ##
==========================================
+ Coverage   45.38%   45.39%   +0.01%     
==========================================
  Files         757      757              
  Lines       85194    85092     -102     
==========================================
- Hits        38664    38630      -34     
+ Misses      40273    40205      -68     
  Partials     6257     6257              

Impacted Files	Coverage Δ
modules/context/context.go	59.55% <ø> (-1.36%)	⬇️
modules/context/repo.go	52.99% <0.00%> (ø)
routers/api/v1/admin/adopt.go	0.00% <0.00%> (ø)
routers/api/v1/notify/user.go	11.62% <0.00%> (ø)
routers/api/v1/repo/key.go	17.77% <0.00%> (ø)
routers/api/v1/repo/release_attachment.go	0.00% <0.00%> (ø)
routers/api/v1/repo/topic.go	47.47% <0.00%> (ø)
routers/web/admin/admin.go	11.41% <0.00%> (ø)
routers/web/admin/emails.go	0.00% <0.00%> (ø)
routers/web/admin/repos.go	0.00% <0.00%> (ø)
... and 63 more

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[6543]

@zeripath can you go fmt for me please?

[6543]

@zeripath I'll merge this for now, if we like to go a interface way as you proposed ... I'm happy to review your pull etc. For now I think this is god to go and we can move things forward ... (It's already cleaner this way & rm useless code)