Abandoning Packages

This PR adds the `abandoned` flag to packages, allowing package owners to mark packages as no longer maintained.
A replecement package can also be suggested and will be displayed along with the warning.
Abandoned packages appear normally but with a warning about its current state.

A un-abandon button is also available to revert.

Next steps: in composer we should make installing these packages throw a warning.

Closes #192.
@stof any particular reason? i can't see much use in tricking people to "un-abandon" packages.