Improve display of custom background images in tip dialog

[zenparsing]

Description

The new rewards tipping dialog uses a different aspect ratio for custom banner images, and when existing banner images are displayed within the new tip dialog, they are cropped and scaled to cover the available space. We should try to scale images with wide aspect ratios more gracefully.

Also, when custom backgrounds have a light color it can become difficult to read the white banner text.

[zenparsing]

After speaking with @codybanxs it was decided to leave the scaling as-is for now and add a missing linear gradient background to custom banner images to improve readability.

[LaurenWags]

Verified passed with

Brave | 1.18.59 Chromium: 87.0.4280.67 (Official Build) dev (x86_64)
-- | --
Revision | 0e5d92df40086cf0050c00f87b11da1b14580930-refs/branch-heads/4280@{#1441}
OS | macOS Version 10.14.6 (Build 18G6042)

Verified test plan from brave/brave-core#6930

1.17.73:

1.18.59:

---

Verification passed on

Brave | 1.18.59 Chromium: 87.0.4280.67 (Official Build) dev (64-bit)
-- | --
Revision | 0e5d92df40086cf0050c00f87b11da1b14580930-refs/branch-heads/4280@{#1441}
OS | Windows 10 OS Version 2004 (Build 19041.630)

Verified test plan from brave/brave-core#6930
1.17.x

1.18.59:

---

Verification passed on

Brave	1.18.62 Chromium: 87.0.4280.67 (Official Build) dev (64-bit)
Revision	0e5d92df40086cf0050c00f87b11da1b14580930-refs/branch-heads/4280@{#1441}
OS	Ubuntu 18.04 LTS

Verified test plan from brave/brave-core#6930
1.17.x

1.18.x