create Get Started section in settings

[rebron]

Test plan

1. Open brave://settings
2. Very top section should be Get Started
3. Items should more or less match the description below (New tab page shows is not done yet- needs to be tracked in Add new tab page settings #2999)
4. Verify Get Started is in the sidebar nav

Description

Create new Get Started section in Settings as the top item.
Label section - 'Get Started'
Label in sidebar - 'Get Started'

This section will include access to Import, Default Browser, New tab page, and On Startup settings.
See design spec.

1. Selecting Import will take you to Import dialog window.
2. Move Default browser setting here. Default Browser will show 'Make Default' button if Brave is not default browser. Display ‘Brave is your default browser’ if Brave has been made the default browser.
3. Create new setting 'New tab page shows' with drop down menu:
   a) Dashboard with images
   b) Homepage
   c) Default search engine
   d) Blank page
4. Move 'On startup' to this section w/ Brave styling.

Design

[srirambv]

+1 from @dfmabbas via #2967 for changing new tab

[rebron]

Created #2999. At minimum for this issue, need import, default browser, and on start up in this section; a regrouping of existing items into this section and styling of those items.

[LaurenWags]

Verified passed with

Brave	0.65.95 Chromium: 74.0.3729.131 (Official Build) beta(64-bit)
Revision	518a41c1fa7ce1c8bb5e22346e82e42b4d76a96f-refs/branch-heads/3729@{#954}
OS	Mac OS X

• Verified test plan from description

• Verification passed on

Brave	0.65.95 Chromium: 74.0.3729.131 (Official Build) beta (64-bit)
Revision	518a41c1fa7ce1c8bb5e22346e82e42b4d76a96f-refs/branch-heads/3729@{#954}
OS	Windows 10 OS Build 17134.523

• Verified the test plan from the description
• Logged The letter s should be in caps in Get started as per the design spec - Follow up of #960 #4499

Verification passed on

Brave	0.65.97 Chromium: 74.0.3729.131 (Official Build) beta(64-bit)
Revision	518a41c1fa7ce1c8bb5e22346e82e42b4d76a96f-refs/branch-heads/3729@{#954}
OS	Ubuntu 18.04 LTS

• Verified test plan from description