Epic: File loading/progress indicators

[jessicaschilling]

This epic is a holding pen for a unified approach to issues in Companion, Web UI and Desktop related to loading and progress indicators. Current issues are:

• Display progress status while importing files ipfs-webui#1206
• Add loading indicator on file preview page ipfs-webui#991
• Loading screen for non-local and redirected content ipfs-companion#727
• Display progress status while adding files (Desktop) ipfs-desktop#1217
• Inform user about potential delays before added content is reachable at public gateway ipfs-desktop#750

In solving/closing these issues, let's make sure that the UI patterns we create are flexible and reusable enough to be applicable to a wide range of loading/progress-related situations outside of these issues.

[jessicaschilling]

@rafaelramalho19 -- here's an epic-labeled issue we can use to discuss work on this effort. Let's touch base after you get a chance to initially assess, so we can work together on UI patterns. Thank you!

[SgtPooki]

@juliaxbow worth a peek if you haven't seen this one yet.

[tinytb]

backlog grooming: will assign this to @juliaxbow after she gets added to the GitHub org (can't assign to her right now)