post_status_form: fix enter key in subject field

This fixes random actions being triggered by the enter key while the
subject field is focused.

When pressing enter, the browser simulates a click on the first "submit"
button it finds in the form.
A submit button is a button without `type="button"` set.
Remediate this by setting the type attribute on all but the "Post"
button.

Additionally, inhibit the enter key in the subject field (ctrl+enter
still works).

src/components/poll/poll_form.vue

@@ -24,6 +24,7 @@
       <button
         v-if="options.length > 2"
         class="delete-option button-unstyled -hover-highlight"
+        type="button"
         @click="deleteOption(index)"
       >
         <FAIcon icon="times" />
@@ -32,6 +33,7 @@
     <button
       v-if="options.length < maxOptions"
       class="add-option faint button-unstyled -hover-highlight"
+      type="button"
       @click="addOption"
     >
       <FAIcon

src/components/post_status_form/post_status_form.vue

@@ -18,6 +18,7 @@
         >
           <button
             class="button-unstyled -link"
+            type="button"
             @click="openProfileTab"
           >
             {{ $t('post_status.account_not_locked_warning_link') }}
@@ -136,6 +137,7 @@
             class="form-post-subject"
             @input="onSubjectInput"
             @focus="focusSubjectInput()"
+            @keydown.exact.enter.prevent
           >
         </EmojiInput>
         <i18n-t
@@ -272,6 +274,7 @@
           <button
             class="emoji-icon button-unstyled"
             :title="$t('emoji.add_emoji')"
+            type="button"
             @click="showEmojiPicker"
           >
             <FAIcon icon="smile-beam" />
@@ -281,6 +284,7 @@
             class="poll-icon button-unstyled"
             :class="{ selected: pollFormVisible }"
             :title="$t('polls.add_poll')"
+            type="button"
             @click="togglePollForm"
           >
             <FAIcon icon="poll-h" />
@@ -290,6 +294,7 @@
             class="spoiler-icon button-unstyled"
             :class="{ selected: subjectVisible }"
             :title="$t('post_status.toggle_content_warning')"
+            type="button"
             @click="toggleSubjectVisible"
           >
             <FAIcon icon="eye-slash" />