Add Tweet button to every Tip panel

[mandar-brave]

Test Plan

brave/brave-core#2195

Add Tweet button to every single Tip panel as part of the tipping process.
When user has successful tipped and land on the final thank you page, add the Tweet button as an option for user to tweet.
Button launched Twitter page in a new tab.

@emerick @NejcZdovc

[LaurenWags]

Verified passed with

Brave	0.67.107 Chromium: 75.0.3770.100 (Official Build) beta(64-bit)
Revision	cd0b15c8b6a4e70c44e27f35c37a4029bad3e3b0-refs/branch-heads/3770@{#1033}
OS	Mac OS X

• Verified test plan from Add support for Twitter tips brave-core#2195
• Verified tweet button appeared on thank you banners after tipping regular sites, YT, Twitch, and Twitter. (screenshots in Tweet is missing hashtag #4555 (comment))
• Note, tweet button is only on one-time tip thank you banner currently, not on monthly/recurring tip thank you banners. Confirmed with team this is expected for now.

Verification passed on

Brave	0.67.108 Chromium: 75.0.3770.100 (Official Build) beta (64-bit)
Revision	cd0b15c8b6a4e70c44e27f35c37a4029bad3e3b0-refs/branch-heads/3770@{#1033}
OS	Windows 10 OS Version 1803 (Build 17134.523)

• Verified test plan from Add support for Twitter tips brave-core#2195
• Verified tweet button appeared on thank you banners after tipping regular sites, YT, Twitch, and Twitter
  (verified all the checks in Tweet is missing hashtag #4555 )

Verification passed on

Brave	0.67.110 Chromium: 75.0.3770.100 (Official Build) beta(64-bit)
Revision	cd0b15c8b6a4e70c44e27f35c37a4029bad3e3b0-refs/branch-heads/3770@{#1033}
OS	Ubuntu 18.04 LTS

• Verified test plan from Add support for Twitter tips brave-core#2195
• Verified tweet button appeared on thank you banners after tipping regular sites, YT, Twitch, and Twitter
  (verified all the checks in Tweet is missing hashtag #4555 )

[binoyxj]

The text included as part of the auto-tweet is actually broken. It's having some gibberish text too.

[LaurenWags]

@binoyxj You're probably experiencing #5229. there's a duplicate user handle and some extra text along with it.

[binoyxj]

@binoyxj You're probably experiencing #5229. there's a duplicate user handle and some extra text along with it.

Yup! https://imgur.com/a/tlit5tw

[LaurenWags]

thanks for confirming @binoyxj !