accounts: share telegram linking modal with other pages

the one on the admin page was a little messed up anyway. Not relevant to
the other linking modals, as the process is different (simpler) on the
admin page.

html/account-linking-matrix.html

@@ -0,0 +1,18 @@
+{{ if .matrixEnabled }}
+<div id="modal-matrix" class="modal">
+    <div class="card relative mx-auto my-[10%] w-4/5 lg:w-1/3">
+        <span class="heading mb-4">{{ .strings.linkMatrix }}</span>
+        <p class="content mb-4"> {{ .strings.matrixEnterUser }}</p>
+        <input type="text" class="input ~neutral @high" placeholder="@user:riot.im" id="matrix-userid">
+        <div class="subheading link-center mt-4">
+            <span class="shield ~info mr-4">
+                <span class="icon">
+                    <i class="ri-chat-3-line"></i>
+                </span>
+            </span>
+            {{ .matrixUser }}
+        </div>
+        <span class="button ~info @low full-width center mt-4" id="matrix-send">{{ .strings.submit }}</span>
+    </div>
+</div>
+{{ end }}