Handling Uphold wallet Connect/Disconnect

[mandar-brave]

@NejcZdovc this is optional for the first phase of user wallets.
Link 1: Just jump to the Uphold wallet
Link 2: Just remove the auth_token and put wallet to disconnect state (if there is an API to de-auth the token, make that call)

[LaurenWags]

Verified passed with

Brave	0.69.121 Chromium: 76.0.3809.132 (Official Build) beta (64-bit)
Revision	fd1acc410994a7a68ac25bc77513d443f3130860-refs/branch-heads/3809@{#1035}
OS	Mac OS X

• Verified test plan from Uphold wallet brave-core#2696
• Verified that Go to my Uphold account opens Uphold dashboard in same tab
• Verified that Disconnect from Brave Rewards disconnects you. Green verified button changes to gray disconnected button. Clicking on it reauthorizes you.
  

Verification passed on

Brave	0.69.123 Chromium: 76.0.3809.132 (Official Build) beta (64-bit)
Revision	fd1acc410994a7a68ac25bc77513d443f3130860-refs/branch-heads/3809@{#1035}
OS	Ubuntu 18.04 LTS

• Verified test plan from Uphold wallet brave-core#2696
• Verified that Go to my Uphold account opens Uphold dashboard in same tab
• Verified that Disconnect from Brave Rewards disconnects you. Green verified button changes to gray disconnected button. Clicking on it reauthorizes you.

Verification passed on

Brave	0.69.128 Chromium: 77.0.3865.75 (Official Build) (64-bit)
Revision	201e747d032611c5f2785cae06e894cf85be7f8a-refs/branch-heads/3865@{#776}
OS	Windows 10 OS Version 1803 (Build 17134.1006)

• Verified test plan from Uphold wallet brave-core#2696
• Verified that Go to my Uphold account opens Uphold dashboard in the same tab
• Verified that Disconnect from Brave Rewards disconnects you. Green verified button changes to gray disconnected button. Clicking on it reauthorizes you.