Config: Search /opt/photoprism for assets and media files

[lastzero]

As a server administrator, I would like PhotoPrism to scan common path names for assets and media files if no explicit folders are specified.

Acceptance Criteria:

• /opt/photoprism/* should be searched too as it is a common installation location.

All currently supported search paths:

• https://github.com/photoprism/photoprism/blob/develop/pkg/fs/dirs.go

Additional originals paths to be searched:

"/photoprism/library/originals",
"/opt/photoprism/library/originals",
"/opt/photoprism/storage/originals",
"/opt/photoprism/originals",
"/library/originals",
"/storage/originals",
"/originals",
"library/originals",
"storage/originals",

Additional import paths to be searched:

"/photoprism/library/import",
"/opt/photoprism/library/import",
"/opt/photoprism/storage/import",
"/opt/photoprism/import",
"/library/import",
"/storage/import",
"/import",
"library/import",
"storage/import",

Assets paths to be searched:

"/photoprism/assets",
"/opt/photoprism/assets",
"~/.photoprism/assets",
"~/photoprism/assets",
"photoprism/assets",
"assets",
"/var/lib/photoprism/assets",