fix(getJoinUrl): Fix function to get join URL for cluster setups in BBB

[GuiLeme]

What does this PR do?

This PR changes the getJoinUrl function so now it is based on the bbbWebBase setting.
One other minor change: Moved this function to be initialized in the initialize method and not the getPlugins method. It won't change much in terms of practical behaviour, but it makes the development a lot easier.

Closes Issue(s)

Closes #54

Motivation

This fix is mostly due to a problem in the plugin-session-share. No changes needed there, though.

[GuiLeme]

One minor comment: The change of setting this method (getJoinUrl) from getPluginApi -> initialize caused one of the plugins developed with the old SDK to break and throw an error.

What happened was:
In SDKv0.0.73 (and older versions too): This method (getJoinUrl) was set by the client (BBB side) with the getPluginApi.

In SDKv0.0.74: This method is set by initialize function, which means it's set from the plugin-side.

So picture the following scenario:
BBB side was updated with SDKv0.0.74, but the plugin wasn't.
This made the plugin not set the getJoinUrl via initialize (because it's outdated and it expects the client to set it), and the BBB side doesn't set it either, because it's already updated, so it expects that the plugin sets it.

The end result is an error saying that getJoinUrl is not a function.

To fix this problem is quite simple: Update the SDK from the plugin side.