Add paragraph about GitHub 2FA and scc commands

- Add link on how to create Tokens using GitHub interface
- Add command to set personal access token globally
- Mention connection via username/password is not possible if 2FA is on
Typo fix
Fix link to 2FA on help.github.com
Fix description of submodule PRs inclusion using scc merge commands
Add note about scc travis-merge limitation
Fix as per @mtbc's suggestion
Use http rather than https links for Sphinx linkchecker
Merge pull request #565 from sbesson/11760_scc_github2fa

Add paragraph about GitHub 2FA and scc commands
Minor fixes
Merge pull request #568 from sbesson/submodule_dependency_fix

Submodule dependency fix