Enable autofocus for Login form

[DavGoeck]

Description

The username input field is now automatically focussed when opening the Login modal if the corresponding configuration option is enabled.

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/geosolutions-it/MapStore2/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

What kind of change does this PR introduce? (check one with "x", remove the others)

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

Issue

What is the current behavior?

#

What is the new behavior?

Breaking change

Does this PR introduce a breaking change? (check one with "x", remove the other)

• Yes, and I documented them in migration notes
• No

Other useful information

[DavGoeck]

As a first attempt at a PR, I'd like to have the Login Form be in focus automatically. While I would want this to be enabled by default I have written it so that it has to be enabled via configuration option in the localConfig.json.
The issue I have is that I don't know an elegant way of loading the config value into the Plugin. Suggestions are welcome!