[C2] Health check: Add critical file permissions checks

[ywarnier]

Add check on access permissions to config/ directory and .env file.
If they are still writeable by the web server user, show them in red.

• ".env is writeable" in red (vs ".env is not writeable" in green)
• "config/ is writeable" in red (vs "config/ is not writeable" in green)

Use language terms %s is writeable and %s is not writeable.

The existence of the public/main/install folder can also be tested, but it should only show orange if it is still present, because not removing it is really not a big deal.

Also add a check on the writeability of var/cache.

@swapnil-up, this seems like a task easy enough to start...

Start by looking for the health-check code in assets/vue/views/admin/AdminIndex.vue, then in IndexBlocksController.php. Don't forget to start in a branch in your fork of this repo, to be able to send a proper PR later on.

[christianbeeznest]

It is checked with this commit 80dd077

Thanks for confirmation.

[ywarnier]

Works well for me. Thanks.