[FIX] mass_mailing: fix tooltip for "Select a theme"

The popover in the mass mailing are not enabled, also the whole CSS of
the website is not used (particularly, the popover CSS is not included).

This commit replace the popover by a tooltip and enable tooltip on
[data-toggle="tooltip"] elements.

second half of:
opw-656693

addons/mass_mailing/static/src/js/mass_mailing_editor.js

@@ -132,6 +132,7 @@ $.summernote.eventHandler.modules.popover.update = function ($popover, oStyle, i
 web_editor.Class.include({
     start: function () {
         var self = this;
+        $('[data-toggle="tooltip"]').tooltip();
         if (location.search.indexOf("enable_editor") !== -1) {
             this.on('rte:start', this, function () {
                 $("#choose_template").off("click").on("click", _.bind(self.on_choose_template, self));

addons/mass_mailing/views/editor_field_html.xml

@@ -30,7 +30,7 @@
         <div id="templates" class="o_not_editable o_css_editor hidden" t-if="enable_editor">
             <h1 class="page-header mt16 email_head panel-primary">
                 Select a theme 
-                <button type="button" class="btn btn-sm btn-default info_popover fa fa-info-circle" data-toggle="popover" data-trigger="focus" title="Help" data-content="Here you can design your own template from scratch by selecting your favorite theme."></button>
+                <button type="button" class="btn btn-sm btn-default fa fa-info-circle" data-toggle="tooltip" data-placement="bottom" title="Here you can design your own template from scratch by selecting your favorite theme."></button>
             </h1>  
             <t t-call="mass_mailing.mass_mail_theme_list"/>