Allow custom certificate point to file location

[Zhwt]

Is your feature request related to a problem? Please describe.
My ISP have some problem and I have to use dns method to get the Let's Encrypt SSL certificate. Since I didn't use Cloudflare, I can't use the built-in renew feature(issue#120). So I wrote a small script to auto renew the certificate every 60 days. But I have to upload the certificate on to the server every time after it is renewed.

Describe the solution you'd like
In the "Add Custom Certificate" page, I can enter a relative/absolute path instead of uploading certificates.

Describe alternatives you've considered
The uploaded certificate files are able to replace.

Additional context
I uploaded a dummy certificate, and tried to create soft link in directories like data/custom_ssl/npm-2 and data/nginx to replace the dummy certificate, after a lot of Internal Error, I ended up with a host that can't be enabled/disabled/deleted. Don't know if the problem is related to wrong file type or an "Internal Error".

[bmcclure]

This change would also allow this to work perfectly with Home Assistant. It has a number of other add-ons that can generate SSL certificates (such as the LetsEncrypt add-on which is able to use DNS validation automatically), and the Home Assistant add-on version of Nginx Proxy Manager does already map Home Assistant's /ssl directory into its container.

The only thing missing is the ability to select a local SSL certificate and key file from the interface instead of having to copy and paste the contents in.

[chaptergy]

Duplicate of #87