Chat Federation: using S2S if available.

* if both local and remote instance have external XMPP connections enabled, the user joins the remote room with his local account
* some code refactoring (builtin.ts)

Note: documentation and settings descriptions are to do.

Related to #112

conversejs/lib/auth.ts

@@ -0,0 +1,65 @@
+interface AuthentInfos {
+  jid: string
+  password: string
+  nickname?: string
+}
+async function getLocalAuthentInfos (authenticationUrl: string): Promise<false | AuthentInfos> {
+  try {
+    if (authenticationUrl === '') {
+      console.error('Missing authenticationUrl')
+      return false
+    }
+    if (!window.fetch) {
+      console.error('Your browser has not the fetch api, we cant log you in')
+      return false
+    }
+    if (!window.localStorage) {
+      // FIXME: is the Peertube token always in localStorage?
+      console.error('Your browser has no localStorage, we cant log you in')
+      return false
+    }
+    const tokenType = window.localStorage.getItem('token_type') ?? ''
+    const accessToken = window.localStorage.getItem('access_token') ?? ''
+    const refreshToken = window.localStorage.getItem('refresh_token') ?? ''
+    if (tokenType === '' && accessToken === '' && refreshToken === '') {
+      console.info('User seems not to be logged in.')
+      return false
+    }
+
+    const response = await window.fetch(authenticationUrl, {
+      method: 'GET',
+      headers: new Headers({
+        Authorization: tokenType + ' ' + accessToken,
+        'content-type': 'application/json;charset=UTF-8'
+      })
+    })
+
+    if (!response.ok) {
+      console.error('Failed fetching user informations')
+      return false
+    }
+    const data = await response.json()
+    if ((typeof data) !== 'object') {
+      console.error('Failed reading user informations')
+      return false
+    }
+
+    if (!data.jid || !data.password) {
+      console.error('User informations does not contain required fields')
+      return false
+    }
+    return {
+      jid: data.jid,
+      password: data.password,
+      nickname: data.nickname
+    }
+  } catch (error) {
+    console.error(error)
+    return false
+  }
+}
+
+export {
+  AuthentInfos,
+  getLocalAuthentInfos
+}