As a user I want to cross-sign my own devices

[dbkr]

• I can use existing sign ins to cross-sign new ones when logging in
• In settings, I can manage my device list
• Unknown sign ins are exposed in useful toasts

In approximate order:

matrix-org/matrix-js-sdk#832

element-hq/element-web#11209
element-hq/element-web#11210
element-hq/element-web#11212
element-hq/element-web#11213 <-- Actually other users but should be done at same time or soon after 11212 (see issue)

[at this stage we basically have cross-signing in labs but with old UI]

element-hq/element-web#11214
element-hq/element-web#11215
element-hq/element-web#11217
element-hq/element-web#11218
element-hq/element-web#11220
element-hq/element-web#11350
element-hq/element-web#11219

[cross-signing now out of labs with new UI for setting up but old UI elsewhere]

[lampholder]

I spun element-hq/element-web#11221 off into #21