Convenience full_name method for User

[graynk]

Returns full name (first + last) if last name is present or just the first name if not present.

[robert-cody]

In many locales format should be "surname name" (which is BTW more useful for sorting), so it's better to make locale as incoming optional parameter and return full_name depending on locale.

[codecov]

Codecov Report

Merging #949 into master will decrease coverage by 0.11%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #949      +/-   ##
==========================================
- Coverage    91.8%   91.69%   -0.12%     
==========================================
  Files         103      103              
  Lines        4040     4044       +4     
  Branches      638      639       +1     
==========================================
- Hits         3709     3708       -1     
- Misses        193      197       +4     
- Partials      138      139       +1

Impacted Files	Coverage Δ
telegram/user.py	86.95% <100%> (+1.24%)	⬆️
telegram/utils/request.py	66.96% <0%> (-0.9%)	⬇️
telegram/bot.py	87.51% <0%> (-0.5%)	⬇️

[graynk]

I don't think this information is in python locale module though?
It seems to have been in CLDR at some point, but not anymore, if I understand it correctly. In any case, I view this as just a simpler version of User.name method that works exactly the same way, only ensuring that returned names are consistent in style (I feel that mentioning some users via "@username" and others via "name" is unintuitive and doesn't look all that well in group chats).

[robert-cody]

@graynk you're right, for some strange reason, there is no such info in Python's locale module.

[Eldinnie]

Looks good to me. Closes #943

[tsnoam]

thank you for your contribution