- Document `optional_plugin_dependencies`
- Also make `converse-headlines` an optional dependency of `converse-dragresize`

updates #805

so that we don't confuse it with the case where it was manual.

Otherwise other plugins don't know whether the message was forwarded or not.

After bookmarking a room for which a nickname is required, return to the nickname form.

The padding didn't appear consistent across browsers.

Otherwise we receive HTML5 notifications for MAM messages because they can't be
identified as such.

since I couldn't find a way to do this with only CSS.

- immediately show chat box (sometimes fetching features takes long)
- add timeout for fetching features
- don't register scroll handler twice for chat rooms.

after the cached messages have been fetched.

when the room features change.

Builds are now themselves UMD modules, so they could be `require`'d like any
other AMD module or they can still be used as old ES5 modules with globals.

Additionally the build is now closured, so that the almond API doesn't pollute
the global context with `define` or `require`. This does however mean that
`require` can no longer be used outside of the build (unless requirej.s is
included directly.