database_widget: tree breaks edit / back button

[nathanweeks]

Enabling database_structure: tree in the ~/.sequenceserver.conf partially breaks the pre-filled search form behavior of the edit/back button described in issue #386 such that the database(s) selected in the original search are "invisibly" selected (they are reused in subsequent queries, but their corresponding checkboxes are unchecked)

To reproduce:

1. Add database_structure: tree to ~/.sequenceserver.conf and start SequenceServer
2. Enter a query sequence in the textarea, select a database, and click the blast* button.
3. In the results page, click the "edit" button. The search page will be presented with the original query sequence prefilled, but no databases selected.
4. Select a different database and click the blast* button

In the results page will show, e.g., "BLASTN: 1 query, 2 databases" despite only one database having been selected in step 3, where the extra database is the one originally selected in step 2.

A preconfigured SequenceServer that illustrates the issue is in this branch named issue-tree. The branch contains a devcontainer.json that has been set up so that when a Codespace is launched on the branch (Code > Create codespace on issue-tree) the SequenceServer container image will be built, started, and served on port 4567 (once SequenceServer is listening on the port, click the Ports tab, then the globe icon for the reverse-proxy URL to access the port in the codespace).