Improving /admin/plugins

[tgxworld (Alan Tan)]

@steve_pd

stuff in core should appear in plugins at all

Plugins that are part of the core repository are still plugins so it should appear in the list.

@steve_pd

so when I see plugins such as presence I know that it has since been merged into core (although the link from /admin/plugins still refers back to the original empty repo)

Fixed in

Update URL for presence plugin.