This moves the apidoc uploading to build-utils/travis-apidoc.sh, which
makes it more pleasant to maintain/read.
Additional enhancements:
- handle all branches and PRs, and provide compare view links as a comment
for the latter, via awesome-robot. This gets skipped for PRs for
forks, because then the Github secret is not available for security
reasons.
- use a merge commit to keep relevant changes and boilerplate together.
- simplified/shorter AWESOME_VERSION override.
- add last commit message of the repo to the apidoc's commit message.
Closes https://github.com/awesomeWM/awesome/pull/358.