Security: proxy-mode is misleadingly global

[Ambrevar]

Our proxy-mode is global, but as a mode it's meant to be buffer-local.
We are dangerously misleading our users here :(

The fix is to use per-buffer WebContexts.
The problem is that on doing so,

1. It generates hundreds of

(process:23271): libsoup-WARNING **: 11:40:55.141: Failed to execute query: table soup_hsts_policies already exists

(process:23271): libsoup-WARNING **: 11:40:55.141: Failed to execute query: database is locked

2. The browser tends to crash after about 1 minute, with

CORRUPTION WARNING in SBCL pid 23231(tid 0x7fd373667700):
Memory fault at (nil) (pc=0x7fd3796214e0, fp=0x7fd3640a4b48, sp=0x7fd3736661c0) tid 0x7fd373667700
The integrity of this image is possibly compromised.
Continuing with fingers crossed.

debugger invoked on a SB-SYS:MEMORY-FAULT-ERROR in thread
#<THREAD "cl-cffi-gtk main thread" RUNNING {1005E73CE3}>:
  Unhandled memory fault at #x0.

The current thread is not at the foreground,
SB-THREAD:RELEASE-FOREGROUND has to be called in #<SB-THREAD:THREAD "main thread" waiting on: #<MUTEX "thread result lock" owner: #<SB-THREAD:THREAD "cl-cffi-gtk main thread" RUNNING {1005E73CE3}>> {1000650083}>
for this thread to enter the debugger.

The 2 issues could be related. Let's get started with the first one.

[Ambrevar]

I've reported the libsoup issue upstream: https://gitlab.gnome.org/GNOME/libsoup/-/issues/192

[Ambrevar]

Merged.