[BUGFIX] Disable extension list during repository update

The extension list is now completely hidden during a repository
update.
Also, the form submit logic has been adjusted and the splash screen
text now shows a spinner icon again.

Resolves: #70098
Releases: master
Change-Id: I2f01c2e5b4a174e79af67500cf7275ba365409ab
Reviewed-on: http://review.typo3.org/43557


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Tested-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>