Remove deprecated models and mobx usage (react-query refactor) (#1934)

* Update login page to use service query

* Update modal to use session instead of store

* Move image sizes cache off store

* Update settings to no longer use store

* Update link-meta fetch to use agent instead of rootstore

* Remove deprecated resolveName()

* Delete deprecated link-metas cache

* Delete deprecated posts cache

* Delete all remaining mobx models, including the root store

* Strip out unused mobx observer wrappers

src/lib/media/image-sizes.ts

@@ -0,0 +1,34 @@
+import {Image} from 'react-native'
+import type {Dimensions} from 'lib/media/types'
+
+const sizes: Map<string, Dimensions> = new Map()
+const activeRequests: Map<string, Promise<Dimensions>> = new Map()
+
+export function get(uri: string): Dimensions | undefined {
+  return sizes.get(uri)
+}
+
+export async function fetch(uri: string): Promise<Dimensions> {
+  const Dimensions = sizes.get(uri)
+  if (Dimensions) {
+    return Dimensions
+  }
+
+  const prom =
+    activeRequests.get(uri) ||
+    new Promise<Dimensions>(resolve => {
+      Image.getSize(
+        uri,
+        (width: number, height: number) => resolve({width, height}),
+        (err: any) => {
+          console.error('Failed to fetch image dimensions for', uri, err)
+          resolve({width: 0, height: 0})
+        },
+      )
+    })
+  activeRequests.set(uri, prom)
+  const res = await prom
+  activeRequests.delete(uri)
+  sizes.set(uri, res)
+  return res
+}