Expand symlinks when loading certificate and key files content

- [X] I am running the latest version
- [X] I checked the documentation and found no answer
- [X] I checked to make sure that this issue has not already been filed

### Issue Details

* **Version of AdGuard Home server:**
* Version: v0.105.2
* **How did you install AdGuard Home:**
* Docker image: adguard/adguardhome:latest
* **How did you setup DNS configuration:**
* Docker on VPS
* **CPU architecture:**
* Digital Ocean basic Docker droplet - 1 GB Memory / 25 GB Disk / SFO3
* **Operating system and version:**
* Docker 19.03.12 on Ubuntu 20.04

### Expected Behavior
My certs are pem files from LetsEncrypt generated by [Swag](https://github.com/linuxserver/docker-swag). Under Settings -> Encryption Settings -> Certificates I should be able to point my certificate path to the `fullchain.pem` file and the private key file path to `privkey.pem`. If there are issues with the certificates themselves, the UI should just say so and not tell me that there's no such file or directory or the cert chain is invalid when the issue is with the key file.

### Actual Behavior
As far as I can tell, the only way to get the files to work at all is to set both fields to point to `priv-fullchain-bundle.pem`. The UI will give various spurious missing file errors and invalid key errors until both fields are set to point to this combined cert/private key file.

### Screenshots

This is what I would expect to put into these fields. These files work perfectly when copied and pasted into the text fields but for some reason don't work when I just point to the files. Notice that the UI is giving a spurious "No such file or directory" error. It gives this for files it does not like regardless of whether or not they actually exist.


Here I have the cert pointing to the file that will eventually work while the private key is still pointing to the `privkey.pem` file. Notice that the UI tells me my cert chain is invalid, even though it is not, and it's also telling me that the privkey file now doesn't exist.


Same with the other way around only now it's the `fullchain.pem` file that it claims doesn't exist.


And here we have it set up in the only way I can find to make it happy. It's only by accepting both files that it stops throwing spurious errors and for some reason it wants the full chain and key pair for the files whereas it's perfectly happy to take them separately when pasting the keys in as text.