"Grants" button should be dynamic and should only appear when the wallet has grants

[kjozwiak]

Test Plan

#1420 (comment)

Description

The Grants button is currently static and is always visible even when there's no grants. This button should be dynamic and should only appear when the wallet has grants. The Grants button should only be visible in the following scenarios:

• user accepts a grant
• user imports a wallet that already has accepted grant(s)

The Grant button should disappear whenever the grants are already used and the list is blank.

Steps to Reproduce

1. launch 0.55.10 Chromium: 70.0.3538.22
2. enable rewards via chrome://rewards
3. you'll notice that the Grants button is visible even though you cannot interact with it

Actual result:

Expected result:

The Grants button should be dynamic and should appear only when there's grants associated with the wallet being used.

Reproduces how often:

100% reproducible using the above STR.

Brave version (chrome://version info)

Brave	0.55.10 Chromium: 70.0.3538.22 (Official Build) beta(64-bit)
Revision	ac9418ba9c3bd7f6baaffa0b055dfe147e0f8364-refs/branch-heads/3538@{#468}
OS	Mac OS X

[LaurenWags]

@NejcZdovc is this sufficient for a test plan:

1. Enable Rewards. Verify grants button is not displayed. Claim a grant. Verify grants button does display.
2. Restore a wallet without any grants in it. Verify Grants button does not display.
3. Restore a wallet with grants in it. Verify Grants button does display.

[NejcZdovc]

@LaurenWags perfect plan

[LaurenWags]

Verified passed with

Brave	0.55.13 Chromium: 70.0.3538.54 (Official Build) beta(64-bit)
Revision	4f8e578b6680574714e9ed3bb9f02922b4dde40d-refs/branch-heads/3538@{#937}
OS	Mac OS X

• Verified steps from test plan

Verification Passed on

Brave	0.55.13 Chromium: 70.0.3538.54 (Official Build) beta (64-bit)
Revision	4f8e578b6680574714e9ed3bb9f02922b4dde40d-refs/branch-heads/3538@{#937}
OS	Windows

• Verified steps from test plan
• Restore a wallet which has funds added from uphold, verified that Grants button does not display

Verification Passed on

Brave	0.55.14 Chromium: 70.0.3538.54 (Official Build) beta(64-bit)
Revision	4f8e578b6680574714e9ed3bb9f02922b4dde40d-refs/branch-heads/3538@{#937}
OS	Linux