LSP login issues

Reported by @regulus79 on Discord:

> A user on the forums mentioned that the sharing platform logs them out immediately, so they can't comment. I tried, and it happens for me too. I can log in, but once I click on a link, it logs me out again

... and @Monospace-V reminded of this solution that was posted on Discord:

> Fixed. Not sure why the cache got blown away.
>
> ```bash
> sudo mkdir -p ~/lmms.io/var/cache/lsp && sudo chown www-data:www-data ~/lmms.io/var/cache/lsp
> ```
>
> If this happens again, there's a chance our git deploy plugin might need that directory whitelisted.

I can confirm that `~/lmms.io/var/cache/lsp` is missing... but what's weird is it's explicitly added in our `hooks/post_deploy` script... Quoting:

https://github.com/LMMS/lmms.io/blob/b6e7afc9f0ce4a5f2a1dc7e54b0733782ae6d7ad/hooks/post_deploy#L9

... which leads me to believe that this script isn't running.... I read the rest of the script and found this line:

https://github.com/LMMS/lmms.io/blob/b6e7afc9f0ce4a5f2a1dc7e54b0733782ae6d7ad/hooks/post_deploy#L4

... which seems to match the symptom described in:
* #377

... so I have reason to believe that `post_deploy` isn't always running after a deployment.

Start with hooks/post_deploy, especially the lines linked in the issue, and compare its behavior with the deployment mechanism implicated by #377. Inspect a deployment to determine whether the hook runs and whether it creates ~/lmms.io/var/cache/lsp with the needed ownership; done means the directory is reliably present after deployment and the logout symptom is resolved.

Written by the indexing model from the issue text.