Brave Rewards needs a 'waiter' message when opting in.

[jasonrsadler]

Description

On first run of browser when enabling Rewards, it is difficult to tell if the browser is responding while waiting for URL responses. Recommending to add a 'please wait' type of message. Also prevent multiple clicking of 'Opt In' (only the first click however, has any effect)

Steps to Reproduce

1. Open Brave on clean profile
2. Enable Rewards
3. Observe length of time from 'Opt In' click until the main Rewards page is displayed.

Actual result:

Rewards page 'lags' a response

Expected result:

The Rewards page should immediately acknowledge that the 'Opt In' button was pressed with a 'waiter' message

Reproduces how often:

Always

Brave version (chrome://version info)

Brave | 0.55.2 Chromium: 70.0.3538.12 (Developer Build) (64-bit)
Revision | ad61584ca32115f953ce656dd75b44f1c502a75d-refs/branch-heads/3538@{#179}
OS | Mac OS X

Reproducible on current release:

Yes

Website problems only:

~~~- Is the issue reproducible on the latest version of Chrome?~~~

### Additional Information
<!--Any additional information, related issues, extra QA steps, configuration or data that might be necessary to reproduce the issue-->

[NejcZdovc]

friendly ping to @jenn-rhim for the spinning icon that we talked about 😃

[srirambv]

Verification Passed on

Brave	0.55.12 Chromium: 70.0.3538.45 (Official Build) beta (64-bit)
Revision	cbdc32e4334458954e9def214d7e5fa1ca1960eb-refs/branch-heads/3538@{#830}
OS	Linux

• Verified Creating wallet spinner is shown when wallet is getting created
• Verified Total line is not shown when amount is added/importing a wallet

Verified passed with

Brave	0.55.12 Chromium: 70.0.3538.45 (Official Build) beta(64-bit)
Revision	cbdc32e4334458954e9def214d7e5fa1ca1960eb-refs/branch-heads/3538@{#830}
OS	Mac OS X

• Verified enabling by either button on initial chrome://rewards page shows the spinner
  
  
• Verified test plan from Welcome page spinner brave-core#580

Verification passed on

Brave	0.55.12 Chromium: 70.0.3538.45 (Official Build) (64-bit)
Revision	cbdc32e4334458954e9def214d7e5fa1ca1960eb-refs/branch-heads/3538@{#830}
OS	Windows 7