'New tab' dashboard images fade in from dark

[petemill]

Fix #5309

Addresses new tab when in 'Dashboard' mode. Sets default background color so that the text is readable without the background having loaded. Waits for image to load completely before fading it in. Also general clean up to remove separate gradient element and double-setting of wallpaper image source.

Note: I noticed after I had done this that this had been attempted before in #6590, but that was abandoned because of the white flash before the tab loads. This minimizes it by further explicitly setting the background color in html before the JS / react components have loaded (although webpack can be configured to output the css as a .css file which can be included in about-newtab.html). The flashes of white that are still present on both tab open and close are from both the background on open, and the window background on close (when no other tab is shown). These will be addressed in #11813.

Here's what it looks like: https://www.dropbox.com/s/qpsk8fk3m5rpx3f/newtab-fade-i5309.mov?dl=0

Here's what it looked like before (with empty profile): https://www.dropbox.com/s/05mz3expvewmcsv/newtab-fade-before-i5309.mov?dl=0

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.

Test Plan:
Automated tests included.

Manual tests include checking that image loads with good network, that fallback image loads with no network and clear cache, and that when settings are set to no images on Dashboard, that the very colorful gradient shows.

Reviewer Checklist:

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[codecov-io]

Codecov Report

Merging #11817 into master will increase coverage by 0.03%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master   #11817      +/-   ##
==========================================
+ Coverage   52.61%   52.65%   +0.03%     
==========================================
  Files         271      271              
  Lines       25715    25673      -42     
  Branches     4104     4100       -4     
==========================================
- Hits        13530    13518      -12     
+ Misses      12185    12155      -30

Flag	Coverage Δ
#unittest	52.65% <66.66%> (+0.03%)	⬆️

Impacted Files	Coverage Δ
js/about/newtab.js	49.68% <66.66%> (-0.07%)	⬇️
js/stores/windowStore.js	27.01% <0%> (+0.29%)	⬆️
app/renderer/components/reduxComponent.js	90.62% <0%> (+6.25%)	⬆️
js/stores/appStoreRenderer.js	100% <0%> (+8.82%)	⬆️

[bsclifton]

This is beautiful- works exactly as expected 😄 👍

Great job!

[bsclifton]

Holding off on merge for this one; we'll likely pull into a future 0.19.x hotfix 😄

[bsclifton]

master 26249f6
0.21.x 663e841
0.20.x a9a1883
0.19.x 5ab40ba