Pass web requests to occ controller if upgrade is needed

[VicDeo]

Description

Related Issue

Fixes owncloud/updater#371

Motivation and Context

503 response to requests initiated by updater

How Has This Been Tested?

Manually.

1. Install 9.1.0
2. Install local update server
3. Patch local update server to serve 9.1.1RC1
4. Patch lib/base.php in 9.1.1RC1
5. Update 'updater.server.url' => '', in config.php to match local update server
6. Trigger update from web UI
   ...
7. Profit!

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[mention-bot]

@VicDeo, thanks for your PR! By analyzing the annotation information on this pull request, we identified @DeepDiver1975, @bartv2 and @LukasReschke to be potential reviewers

[VicDeo]

@PVince81 please have a look

[VicDeo]

Setting severity according to the issue

[VicDeo]

Stable9.1: #26048
Stable9: #26049

[PVince81]

Tested, works 👍

[lock]

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