Tweak handling of strain, Pan and subtree export

[twalsh-ebi]

Description

This PR would:

• add a GENE_TREE_CONSTANTS function in the EnsEMBL::Web::Constants module;
• make use of the GENE_TREE_CONSTANTS function to pass appropriate cdb, action, component and strain parameters in "Tree or Alignment" or "Sequences" export URLs in gene-tree node ZMenus;
• ensure that node and strain parameters are passed when the Back button is used in a gene-tree download modal window.

The effects of this would be that:

• clicking the "Tree or Alignment" or "Sequences" subtree export options in a strain, Pan or Metazoa gene-tree node Zmenu would lead to the tree/alignment or sequences (respectively) of the subtree rooted at the given node;
• when previewing data for a strain/subtree, if a user clicks the Back button and clicks the Preview button again, they will return to a preview of the given strain/subtree. Currently, in this scenario, the user is presented with data for the full default gene tree.

Views affected

This change would affect gene-tree export previews/downloads for internal tree nodes, strain gene trees, and internal nodes of strain/Pan gene trees.

In conjunction with eg-web-metazoa PR 42, it would also benefit gene-tree export previews/downloads for internal nodes of Metazoa gene trees.

Example of a Newick preview for a subtree (node_id: 3002982674) of the cultivar gene tree of Rice gene Os05g0421750:

Possible complications

None expected.

Merge conflicts

None detected.

Related JIRA Issues (EBI developers only)

N/A