[REF] web_editor, *: review color theming system

* web, website

- Introduce gray color palettes (needed for themes migration)

- Synchronize BS4 color maps with individual variables (see comments
 about this in the code).

- Review alpha/primary, beta/secondary matching:

Before this commit, we decided that the common way to define a color
palette was defining primary, secondary, gamma, delta and epsilon.
alpha and beta were then forced to primary and secondary without other
possibility.
The new system makes more sense:
1) define alpha, beta, gamma, delta and epsilon
2) primary and secondary will automatically be set to your alpha and
  beta (allowing to style the default UI with BS4-independant
  variables)
3) if you are not happy with (2), you can define primary / secondary
  in your color palette so that they are not automatically set to
  alpha / beta

This commit also changes what classes the editor uses. Background colors
and text colors will now use alpha/beta/gamma/delta/epsilon (not primary
and secondary anymore). For buttons, all the possibilities are suggested
but color duplicates are hidden (so if your primary and alpha are equal,
only one button color is suggested).

addons/web_editor/views/editor.xml

@@ -69,7 +69,6 @@
 <template id="_assets_backend_helpers" inherit_id="web._assets_backend_helpers">
     <xpath expr="//link[last()]" position="after">
         <link rel="stylesheet" type="text/scss" href="/web_editor/static/src/scss/bootstrap_overridden.scss"/>
-        <link rel="stylesheet" type="text/scss" href="/web_editor/static/src/scss/bootstrap_overridden_backend.scss"/>
     </xpath>
 </template>
 
@@ -154,8 +153,8 @@
 <template id="web_editor.colorpicker" name="Color-Picker">
     <colorpicker>
         <div class="o_colorpicker_section" data-name="theme" data-display="Theme Colors" data-icon-class="fa fa-flask">
-            <button data-color="primary"></button>
-            <button data-color="secondary"></button>
+            <button data-color="alpha"></button>
+            <button data-color="beta"></button>
             <button data-color="gamma"></button>
             <button data-color="delta"></button>
             <button data-color="epsilon"></button>