Fix proxy addresses for embedders and NLTK

[PrimozGodec]

Issue

• get_proxy from core Orange is not working properly - it document affects embeddings
• nltk needs explicit port address in proxy URL

Description of changes

• temporary add the same function as in [FIX] Embedders - fix proxies, default on http, tests orange3#6028 to text util core Orange is released
• for NLTK add a port to the address if missing. Guess port based on the scheme. If any other port is needed user should specify it

Includes

• Code changes
• Tests
• Documentation

[codecov-commenter]

Codecov Report

Merging #867 (3104d28) into master (c05b4fb) will increase coverage by 0.04%.
The diff coverage is 86.20%.

@@            Coverage Diff             @@
##           master     #867      +/-   ##
==========================================
+ Coverage   76.82%   76.87%   +0.04%     
==========================================
  Files          85       85              
  Lines       11942    11965      +23     
  Branches     1877     1880       +3     
==========================================
+ Hits         9175     9198      +23     
+ Misses       2463     2462       -1     
- Partials      304      305       +1