Improve new user (no WebID) workflow

[dmitrizagidulin]

Improve the user experience for first-time users who are encountering a Solid app (for example, have downloaded and ran a Solid server, and Warp is the default app it launched with).

At the moment, the current workflow is as follows.

User visits https://linkeddata.github.io/warp/, and encounters a text box, "Please provide a location for the LDP server:". Several issues here:

• No explanatory text for what Warp is.
• No explanation for why they need to specify an LDP server, where those could be found, or how to get an account.

Proposed solution:

1. Add a sentence of explanation "Warp is a client-side file browser for the Solid platform" (and linked to Solid). (Or LDP, whichever).
2. Add wording to the effect of "You will need a WebID account at one of the following storage providers", and link to something like https://solid.github.io/solid-idps/
3. Provide a link (or a popup tooltip) on 'What is a WebID'

User encounters the Warp app as a result of launching a Solid server (which specifies Warp as a default app). Issues here:

• User encounters an in-browser popup that prompts them for a certificate. Since most users don't have one (and don't know what they are), they are likely to hit Cancel (or just not have a certificate to select).
• If a user hits cancel, the app flashes a brief notification, "Could not fetch dir listing, HTTP 401" but that's all.

Proposed solution:

1. If the user is not authenticated (not signed in), display a 'Sign in with WebID' button.
2. If the user hits Cancel on the certificate popup, display explanatory text. Something like "Warp requires that you sign in and have a valid storage provider account". Link to the list of storage providers, as above.

For example, here's a screenshot for how http://webid.im/ handles that same situation. (Note the helpful 'What is a WebID' doc tooltip link).

Here's a similar UI from solid/inbox:

[dmitrizagidulin]

See also solid-contrib/solid-signup#3 - related issue