Ability to preview and upload logo and background images

Re: pleroma#1516 (closed)

Complementary BE MR pleroma!2388 (merged)

Screenshots:

mobile

src/api/mediaUpload.js

+import { getToken } from '@/utils/auth'
+import { baseName } from './utils'
+
+const UPLOAD_URL = '/api/v1/media'
+
+export function uploadMedia({ formData, authHost }) {
+  const url = baseName(authHost) + UPLOAD_URL
+
+  return fetch(url, {
+    body: formData,
+    method: 'POST',
+    headers: authHeaders()
+  })
+    .then((data) => data.json())
+}
+
+const authHeaders = () => {
+  return { 'Authorization': `Bearer ${getToken()}` }
+}