[PLT-1249] Add close button 'x' to the right of a link preview (#7017)

* add close button 'x' to the right of a link preview

* Updating webhook UI

* UI improvements for close button

* Adding hover state

* Making the close button visible on mobile

* previews are permanently disabled/closed for that link

* make post as required props

* fix JS error of undefined

* fix update issue both at center and RHS view

webapp/utils/utils.jsx

@@ -653,6 +653,8 @@ export function applyTheme(theme) {
         changeCss('.app__body .post.post--comment.other--root.current--user .post-comment, .app__body .more-modal__list .more-modal__row, .app__body .member-div:first-child, .app__body .member-div, .app__body .access-history__table .access__report, .app__body .activity-log__table', 'border-color:' + changeOpacity(theme.centerChannelColor, 0.1));
         changeCss('@media(max-width: 1800px){.app__body .inner-wrap.move--left .post.post--comment.same--root', 'border-color:' + changeOpacity(theme.centerChannelColor, 0.07));
         changeCss('.app__body .post.post--hovered', 'background:' + changeOpacity(theme.centerChannelColor, 0.08));
+        changeCss('.app__body .attachment__body__wrap.btn-close', 'background:' + changeOpacity(theme.centerChannelColor, 0.08));
+        changeCss('.app__body .attachment__body__wrap.btn-close', 'border-color:' + changeOpacity(theme.centerChannelColor, 0.2));
         changeCss('@media(min-width: 768px){.app__body .post:hover, .app__body .more-modal__list .more-modal__row:hover, .app__body .modal .settings-modal .settings-table .settings-content .section-min:hover', 'background:' + changeOpacity(theme.centerChannelColor, 0.08));
         changeCss('.app__body .more-modal__row.more-modal__row--selected, .app__body .date-separator.hovered--before:after, .app__body .date-separator.hovered--after:before, .app__body .new-separator.hovered--after:before, .app__body .new-separator.hovered--before:after', 'background:' + changeOpacity(theme.centerChannelColor, 0.07));
         changeCss('@media(min-width: 768px){.app__body .suggestion-list__content .command:hover, .app__body .mentions__name:hover, .app__body .dropdown-menu>li>a:focus, .app__body .dropdown-menu>li>a:hover', 'background:' + changeOpacity(theme.centerChannelColor, 0.15));