Add option ":databases_widget: tree" to display databases in tree format

[loraine-gueguen]

Introduced in release 2.0.0 :

We have introduced an optional, experimental widget to display databases in tree format - very helpful if you have a long list of databases. The feature was contributed by @Bjoernsen (wurmlab/sequenceserver#520) and is similar to what was previously produced by lepbase (wurmlab/sequenceserver#307). See wurmlab/sequenceserver#520 for details. To activate the widget, add ":databases_widget: tree" to your SequenceServer config file (which is ~/.sequenceserver.conf by default). The tree widget additionally allows linking databases to external page (how to do so will be documented on the website in due time)

[loraine-gueguen]

https://sequenceserver.com/doc/#tree-widget