Revert back to Bootstrap v4.6.1

[chalin]

Closes #880

I did:

$ pushd assets/vendor/bootstrap
$ git checkout v4.6.1 
Previous HEAD position was 6ffb0b48e Release v4.6.0. (#32726)
HEAD is now at 043a03c95 Release v4.6.1 (#35272)
$ popd
$ npm run build
...
$ (cd public && git diff -- . ':(exclude)*.map')

diff --git a/scss/main.css b/scss/main.css
index d1fec32..e3f57a3 100644
--- a/scss/main.css
+++ b/scss/main.css
@@ -12,7 +12,7 @@ See https://github.com/twbs/bootstrap/pull/23260
 @import url("https://fonts.googleapis.com/css2?family=Tajawal:wght@300;400;500;700&display=swap");
 @fa-font-path :        "../webfonts";
 /*!
- * Bootstrap v4.6.0 (https://getbootstrap.com/)
+ * Bootstrap v4.6.1 (https://getbootstrap.com/)
...