Currently clicking some web installer buttons, like 'Lock bootloader',
can interrupt flashing and some, like 'Flash release', can cause
multiple instances of the "flashing" process to initiate. This commit adds
support for enabling/disabling web installer buttons and disables buttons
that might interrupt flashing while flashing.
Clicking the 'Download release' button while a download is in progress
can cause multiple downloads to initiate, so disable the button during
downloads and enable it afterwards.
0f37181876