Add font size options

To test this, you will need to have access to the browser dev tools, either in the Moodle desktop app or by using USB debugging on a device/emulator. Log in to the app Go App Settings > General in the main menu You should see a "Text size" option with a 3-part segment control, showing a small, medium and large "A". The smallest should be active. Check the size of the "If enabled, a text editor will..." text on this page.  The computed size should be 14px. Press the middle-sized "A". The text should get bigger. Check the size of the "If enabled, a text editor will..." text on this page.  The computed size should be 17px (possibly with a tiny rounding error). Press the largest-sized "A". The text should get bigger. Check the size of the "If enabled, a text editor will..." text on this page.  The computed size should be 21px. Press the smallest "A" The text should return to the original size. Check the size of the "If enabled, a text editor will..." text on this page.  The computed size should be 14px. Press the middle-sized "A". The text should get bigger. Restart the app. When the app reloads, the text should still be at the middle size. Go App Settings > General in the main menu The middle-sized "A" should be selected.