Add separate Procfile with Webpack assets process

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Update INSTALL doc to use foreman

[ci skip]

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Tell Teatro to use development Procfile

[ci skip]

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Merge pull request #2200 from opf/feature/dev-procfile

Add a development Procfile, document in INSTALL