Add release branch maintenance tasks

[chillu]

We usually branch off micro releases (x.y.z) once they go into alpha/beta/rc stage,
in order to avoid code-freezing the minor release branches (x.y).
This is a manual process which should be wrapped in phing tasks.
Note: Some of this is already present in the archive and create-composer-lock tasks.

create-release-branch <branch>

• Create and check out branch (ensure it has the right notation)

rc-release <tag>

• Ensure correct branch (micro release)
• Add @stable to composer core requirements
• Create or update composer.lock file (commit+push)
• Tag release and push tags

stable-release <tag>

• Ensure correct branch (micro release)
• Create or update composer.lock file (commit+push)
• Check out base release branch
• Tag release and push tags
• Merge back micro release branch
• Remove @stable from composer core requirements
• Remove micro release branch
• Remove composer.lock (commit+push)