Implement storage context dialogue

Implement the storage dialogue. The design for it is at the bottom of this design sheet. The text in the storage context tile is implemented in #1352 (closed).

Open questions / differences from the design:

• ‘Potential additional downloads’ are not implemented because we’ve got no data for that (and that’s not going to happen for 41) => deferred to #1393
• The description text at the top of the dialogue isn’t implemented because it’s basically a copy of the ‘potential additional downloads’ size. Let me know if it should be added anyway
• ‘Addons’ are not implemented because that would be too hard at the moment (i.e. not possible for 41), although the data should be available. It strikes me as a bit odd, though, that the content of the dialogue would change based on which addons are ticked on the details page. I don’t know if users would discover that easily => deferred to #1393
• The ‘Manage Storage’ button; it should be possible to implement for 41, but would be less risky if we punted it out to a follow-up MR (since other tasks due for the UI freeze are more important) => #1394 (closed)
• Haven’t styled the sizes with the units smaller than the number, as that’s a bit tricky and would be inconsistent with the storage context tile in its current form. The styling could be applied uniformly in a follow-up MR, but I can’t think of a good way of splitting the output from g_format_size() to get separate formatted size and unit strings, so it might not be feasible. => deferred to #1391 (closed)
• The ‘Required Dependencies’ row is still shown, even if it’s 0 bytes. Should it be hidden if zero?
• The total download size (shown in the lozenge at the top of the dialogue) doesn’t match what’s shown on the context tile because the download size of the dependencies is added to it in the dialogue, but not in the context tile. That matches the designs, but seems potentially confusing. How should we handle that? => deferred to #1393

Closes #1353 (closed)