This change provides some customization for messages posted to I Done This.
Event types can be filtered so that only some kinds of activity get posted,

while others do not. For example, the "Reply published" event can be disabled

to reduce noise in I Done This from every single reply getting published.
The posted messages can also be customized independently for every event type.

An empty message causes a fallback to the default format string, which is shown

below the text field for reference. Parameters such as the review request ID,

summary, and URL are substituted into placeholder strings, or omitted from the

final message. The syntax for placeholders is from string.Template, which is

limited and safer compared to other kinds of string formatting.
New unit tests were added to verify the added functionality, and some existing

tests were updated to test individual event type filtering.

All unit tests pass.
Manually tested event type filtering. Choosing "All events" posts all activity

as before. Choosing a subset of event types only posts those events.
Also specified some custom message template strings and verified that they get

formatted and posted to I Done This, or cause appropriate validation errors.