treewide: change license to MPL-2.0

Adjust the license to be able to upstream adjustments from this
repository to upstream Firefox. I've asked all contributors if they are
fine with the license change in the related issue.

Fixes: https://gitlab.com/postmarketOS/mobile-config-firefox/-/issues/31

src/userChrome/appMenu.css

@@ -1,5 +1,5 @@
 /* Copyright 2021 Oliver Smith
- * SPDX-License-Identifier: GPL-3.0-or-later */
+ * SPDX-License-Identifier: MPL-2.0 */
 
 @media (max-width: 700px) {
     /* Spawn the menu above the navigation bar (now that we've moved it to the

src/userChrome/browser.css

@@ -1,5 +1,5 @@
 /* Copyright 2021 plata
- * SPDX-License-Identifier: GPL-3.0-or-later */
+ * SPDX-License-Identifier: MPL-2.0 */
 
 /* Move navigation bar to bottom */
 @media (max-width: 700px) {

src/userChrome/editBookmarkPanel.css

@@ -1,5 +1,5 @@
 /* Copyright 2021 Oliver Smith
- * SPDX-License-Identifier: GPL-3.0-or-later */
+ * SPDX-License-Identifier: MPL-2.0 */
 
 @media (max-width: 700px) {
     /* The only way I (ollieparanoid) found to make this not glitchy, was to

src/userChrome/findbar.css

@@ -1,5 +1,5 @@
 /* Copyright 2021 Oliver Smith
- * SPDX-License-Identifier: GPL-3.0-or-later */
+ * SPDX-License-Identifier: MPL-2.0 */
 
 @media (max-width: 700px) {
     .findbar-container {

src/userChrome/popups.css

@@ -1,5 +1,5 @@
 /* Copyright 2021 Oliver Smith
- * SPDX-License-Identifier: GPL-3.0-or-later */
+ * SPDX-License-Identifier: MPL-2.0 */
 
 @media (max-width: 700px) {
     /* Hack to prevent popups from flickering around. It looks like e.g. the

src/userChrome/root.css

@@ -1,5 +1,5 @@
 /* Copyright 2021 Oliver Smith
- * SPDX-License-Identifier: GPL-3.0-or-later */
+ * SPDX-License-Identifier: MPL-2.0 */
 
 /* Reduce minimum window width */
 :root {

src/userChrome/tabmenu.css

@@ -1,5 +1,5 @@
 /* Copyright 2021 Peter Mack, Oliver Smith
- * SPDX-License-Identifier: GPL-3.0-or-later */
+ * SPDX-License-Identifier: MPL-2.0 */
 
 @media (max-width: 700px) {
     /* Even when not in private browsing mode, reserve space to the right of

src/userChrome/urlbar.css

@@ -1,5 +1,5 @@
 /* Copyright 2021 Oliver Smith
- * SPDX-License-Identifier: GPL-3.0-or-later */
+ * SPDX-License-Identifier: MPL-2.0 */
 
 /* Reduce minimum window width */
 #urlbar-container {