Allow [Foo] Done something syntax for servers log

Summary:
Log entries syntax is "[component] message".

If a component is known, such syntax is directly allowed.
If a component is unknown, the user is invited to prepend .+log.

Test Plan:
Tested with 21:44 < Dereckson> [Eglide] Prune libapache2-mod-php7.1.

Unit tests are provided for is_known_component.

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1366