Add a shares storage provider for accessing shares in /home

[aduffeck]

This PR implements a new storage provider which exposes all shares the user has access to.

When mounted to /home/Shares it makes the magic in the gateway code superfluous. See https://github.com/aduffeck/ocis/commits/sharestorageprovider for an example implementation of that scenario.

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[lgtm-com]

This pull request introduces 1 alert when merging 5c44e80 into 567ec47 - view on LGTM.com

new alerts:

• 1 for Useless assignment to local variable

[lgtm-com]

This pull request introduces 1 alert when merging 2c2e719 into df52c78 - view on LGTM.com

new alerts:

• 1 for Useless assignment to local variable

[lgtm-com]

This pull request introduces 1 alert when merging f447a77 into df52c78 - view on LGTM.com

new alerts:

• 1 for Useless assignment to local variable

[ishank011]

@aduffeck @butonic would it also be a good idea to move the shares directory out to its separate mount instead of restricting it inside the home directory in this PR as well? I started on that in #1584. The tests fail because in ocdav, we prepend /home to all incoming requests including /Shares/..., so such requests would need to be handled.

[aduffeck]

@ishank011 that is exactly what we are currently doing in #1846 (which kinda supersedes this pr, so I will close this one now). The shares are currently mounted to /home/Shares but it could be any other path as well.