Add a "create a similar rollup" link in rollup PR body

[oliviacrain]

Add a link to the bottom of generated rollup PR bodies which generates a similar rollup on the queue page. Adds a new web.base_url property in the configuration file.

The similar rollup link generator takes all PRs that are successfully merged in a rollup and puts their numbers (comma-delimited) into a query string with the field name prs. That query string is parsed by the queue page handler, which passes that information to the templating engine. The templating engine then inserts an appropriate state for each PR's checkbox on the queue page.

Tested functionality on a personal repo. I specifically tested that (a) the query string parsing is fault-tolerant and that (b) the checkbox state calculation does not cause unapproved or try-pending PRs to be checked.

Fixes #89.

[Mark-Simulacrum]

I would prefer to avoid putting these URLs in PR bodies if we don't automatically strip them out when adding the PR to the commit log. Maybe we could add that filtering? See e.g. #100 for an example of such.

That said I'd want feedback from people doing rollups on the regular if this is a good solution -- cc @RalfJung @Dylan-DPC @jonas-schievink (we really need a pingable team here)

[RalfJung]

It's been a while since I did rollups but this looks great. :-)

[pietroalbini]

An easy way to suppress the link in merge commit bodies would be to wrap the link between <!-- homu-ignore:start --> and <!-- homu-ignore:end -->, and strip everything between those two HTML comments when preparing the merge body.

[Dylan-DPC-zz]

I would prefer to avoid putting these URLs in PR bodies if we don't automatically strip them out when adding the PR to the commit log.

I agree with this

[oliviacrain]

Added a mechanism for stripping out arbitrary text in the auto merge and rollup merge commit bodies, using @pietroalbini's suggestion.

[pietroalbini]

Thanks! Let's see how this goes!