refactor: replace most external libraries icons with coreui icons

Not replaced:
- brand buttons,
- payment methods in dashboard

src/views/pages/Register.vue

@@ -10,24 +10,24 @@
                 <p class="c-text-muted">Create your account</p>
                 <CFormInput
                   placeholder="Username"
-                  prepend="<i class='icon-user'></i>"
+                  prependHtml="<i class='cui-user'></i>"
                   autocomplete="username"
                 />
                 <CFormInput
                   placeholder="Email"
-                  prepend="@"
+                  prependHtml="@"
                   autocomplete="email"
                 />
                 <CFormInput
                   placeholder="Password"
                   type="password"
-                  prepend="<i class='icon-lock'></i>"
+                  prependHtml="<i class='cui-lock-locked'></i>"
                   autocomplete="new-password"
                 />
                 <CFormInput
                   placeholder="Repeat password"
                   type="password"
-                  prepend="<i class='icon-lock'></i>"
+                  prependHtml="<i class='cui-lock-locked'></i>"
                   autocomplete="new-password"
                   class="c-mb-4"
                 />