Update URL links from external sources

User story:
As a Jalview user I want to navigate to one or more database web pages for a specific alignment, so that I can gather more information about the alignment. I can navigate to a preferred database directly from the sequence, or I can select from a set of other databases.
 
Test cases:
Make sure all the database references acquired by db refs functionality are available to the user as links.
Make sure that preset database links (in preferences or via the project file) are always available to the user, for any alignment.
Make sure that references which operate as a group (GO references, what else?) are linked to a single web page.
Make sure that links are up to date with current known database URLs.
Make sure that the user can set their own default database.

It is likely that this change will require an update to the URL links GUI functionality. Picking up suggested changes for the GUI from JAL-2282:
- option to reorder links in the URL links dialog (this enables user to set their own default database easily)
- ability to insert $SEQUENCE_ID$ or $DB_ACCESSION$ token directly into URL link via button