Support GitHub Issues and drop b.p.o

[Mariatta]

Expect gh- issue number in titles.
Remove logic for adding bpo url in the body.
Validate that the GH- issue is indeed an issue, not PR

Closes #418

[codecov]

Codecov Report

Merging #420 (d8892fe) into master (2aee86a) will not change coverage.
The diff coverage is 100.00%.

❗ Current head d8892fe differs from pull request most recent head cd97d6b. Consider uploading reports for the commit cd97d6b to get more accurate results

@@            Coverage Diff             @@
##            master      #420    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           10        20    +10     
  Lines         1300      1689   +389     
==========================================
+ Hits          1300      1689   +389     

Impacted Files	Coverage Δ
bedevere/__main__.py	100.00% <100.00%> (ø)
bedevere/gh_issue.py	100.00% <100.00%> (ø)
tests/test_gh_issue.py	100.00% <100.00%> (ø)
tests/test_prtype.py	100.00% <0.00%> (ø)
tests/test_filepaths.py	100.00% <0.00%> (ø)
bedevere/news.py	100.00% <0.00%> (ø)
bedevere/close_pr.py	100.00% <0.00%> (ø)
bedevere/stage.py	100.00% <0.00%> (ø)
bedevere/follow_up.py	100.00% <0.00%> (ø)
bedevere/filepaths.py	100.00% <0.00%> (ø)
... and 4 more

[brettcannon]

I'm assuming this is mostly a copy-and-paste job from the bpo code.