login button doesn't show that anything is happening

**Describe the bug**

The `POST /_matrix/client/r0/login` endpoint may take some time to respond. During this time, there is no feedback to the user that anything has going on, so it's unclear if the user action has been registered, the application has had an error, or what's going on in general.

**To Reproduce**
Steps to reproduce the behavior:
1. Enter login details
2. Click 'Login'

**Expected behavior**

A spinner? or greying out the button?

**Screenshots**

**Desktop (please complete the following information):**
- OS: linux
- Browser: chrome

**Additional context**

Reproduce the delay after submitting login details to POST /_matrix/client/r0/login in Chrome on Linux and inspect the login UI entry point. Done means the button visibly indicates that the request is in progress, such as with a spinner or disabled styling, and the behavior can be verified during a slow response.

Written by the indexing model from the issue text.