Add a light theme (#7027)

* Add a light theme to Mastodon

This add a second default theme to Mastodon. This new theme is supposed to be a light version of the dark original one.

* Update locales

Update the English and French locales.

* Change capitalization of hexadecimal triplets

Change capitalization of hexadecimal color codes to match Code Climate's recomandation.

* Add variable

Add a new variable to use instead of hardcoding a color. Change made to match Code Climate's recomandation.

* Use Mastodon branding palette

Replace the colors previously used by the ones found in Mastodon branding palette.
The resulting theme is lighter than the previous version.

* Make the overlay background white

Make the overly background used on on sensitive medias white instead of black

* Change the color used on the envelop icon

Change the color used on the envelop icon for a darker one. The same color is now used on both the envelop icon and the padlock icon.

* Add contrast

Add contrast to various places inside the main interface and the profile pages.

* Change the text color used in the compose form

Change the text color used in the compose form for a darker one.

* Make the code easier to read

Add some blank lines to make the code easier to parse for a human eye.

* Change columns' background default colors

Change columns' background default colors, making the composition column the darker one.

* Change the color of the log in button

Change the log in button's text color to make it more readable.

* Fix the color of the boost buttons on the landing page

The disabled boost buttons on the landing page are now of the same color that the other disabled buttons.

* Change the colors used in the dropdown menu

Make the dropdown menu light instead of dark.

config/locales/en.yml

@@ -777,6 +777,7 @@ en:
   themes:
     contrast: High contrast
     default: Mastodon
+    mastodon-light: Mastodon (light)
   time:
     formats:
       default: "%b %d, %Y, %H:%M"

config/locales/fr.yml

@@ -700,6 +700,7 @@ fr:
   themes:
     contrast: Contraste élevé
     default: Mastodon
+    mastodon-light: Mastodon (clair)
   time:
     formats:
       default: "%d %b %Y, %H:%M"

config/themes.yml

@@ -1,2 +1,3 @@
 default: styles/application.scss
 contrast: styles/contrast.scss
+mastodon-light: styles/mastodon-light.scss