Use config api for network form help texts and defaults

[edlerd]

Done

• Use config api for network form help texts and defaults
• unify network form key handling

Fixes WD-7391

QA

1. Run the LXD-UI:
   • On the demo server via the link posted by @webteam-app below. This is only available for PRs created by collaborators of the repo. Ask @mas-who or @edlerd for access.
   • With a local copy of this branch, run as described here.
2. Perform the following QA steps:
   • check create/edit managed networks and ensure the help texts are displayed correctly

[webteam-app]

Demo starting at https://lxd-ui-695.demos.haus

[mas-who]

LGTM, also tested user custom keys when editing networks and it did not regress 👍