Gantt view: use initialOptions to save & restore view state on navigation

[markgrahamdawson]

These changes build on the work undertaken to save & restore workspace layout on navigation #1664

Which is part of a larger issue #662

There was an equivalent pull request for the log view #1688
There is an equivalent pull request for the tree view which this builds on: #1705
There is an equivalent pull request for the table view which this builds on: #1711
There is an equivalent pull request for the graph view which this builds on: #1717
There is an equivalent pull request for the analysis view which this builds on: #1744

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• CHANGES.md entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.

[MetRonnie]

I've resolved the conflict (replaced CHANGES.md edit with towncrier fragment)

[MetRonnie]

@ChrisPaulBennett We don't need a full review from you, but just want to make you aware of this change to the Gantt code