RN: iOS13/Android Q Dark theme for Server URL and Login screens

Description

Server screen and login screen should have dark theme versions given the new dark themes in iOS and Android Q

Server URL screens: https://invis.io/H2T1YTSSVPT#/374196008_Project_Details
Login Screens: https://invis.io/H2T1YTSSVPT#/374196026_Project_Details_2

QA Test Steps

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Reporter

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Mana

1

Assignee

Mattermost Community

QA Assignee

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Reporter

Eric Sethna

Fix versions

Mattermost Team

Mobile

Sprint

None

Labels

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QA Testing Areas

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Components

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