[FIX] mail: fix composer css style

Revision of https://github.com/odoo/odoo/commit/c778de44c7cd9b7ef5a9a91738164ab0ebd0ca06

The commit above changed the composer in order to add colored text for mentions
(instead of textarea, it uses a div with contenteditable).

Some tweaks were made in order to preserve the css style of the composer, but
there were some leftovers:

    - placeholder "Write Something" should be more transparent
    - cursor hover on placeholder should be of type text
    - text is not vertically centered with the input container
    - there should be no focus border when the composer has the focus
    - "Send" button is vertically misaligned by 1 pixel off in discuss

This commit solves all of the above problems.

As a result, the css style of the composer should look almost similarly to the
one in v11.

addons/mail/static/src/less/composer.less

@@ -29,11 +29,15 @@
     .o_composer {
         .o_composer_input[contenteditable=true]:empty:before {
             content: attr(placeholder);
-            opacity: 0.75;
+            opacity: 0.4;
         }
         .o_composer_input {
+            border: none;
+            box-shadow: none;
+            cursor: text;
             display: inline-block;
             height: 100%;
+            padding: 0px 0px @o-mail-chatter-gap;
             white-space: pre-wrap;
             word-break: break-all;
         }
@@ -156,8 +160,6 @@
             }
             .o_composer_text_field {
                 .o-flex(1, 1, 50%);
-                margin: -4px 7px 5px 0px;
-                padding: 2.5px 0px 0px 5px;
             }
             .o_chatter_composer_tools {
                 .o-flex(0, 1, auto);