v5.99 does not respect edge clearance when opening v5.1 board

Description

Regarding #6426 (closed): v6 should correctly import "Copper edge clearance" constrain from v5.1 pcb. Or at least, warn user about a change.

Steps to reproduce

1. Create board with v5.1. In this case, edge clearance is 0.4 mm from three sides and 2 mm from "fattier" side: noname.kicad_pcb;
2. Refill all area occupying zone with 0 mm clearance;
3. Open the same board with v6, refill;
4. You will get 0 mm clearance from board edges, produsing different gerbers and user can be unaware of this.

Proposed solution:

1. "Copper edge clearance" should be automatically recalculated and set to "edge.cuts_outline_line_thicknesss / 2", in case v5.1 pcbs edge.cuts consists only of closed chape lines of the same thickness;
2. Do not even try to import "Copper edge clearance" for case with different linethicknesses, like in this case. Continuously show warning infobar, that board clearance must be set, until user will explicitly set "Copper edge clearance" to non zero value.

Keep in mind, that edge.cuts layer must be checked for sanity in step 1: #6410 (comment 449717093).

KiCad Version

Application: Pcbnew

Version: (5.99.0-7076-g0e44f5128c), release build

Libraries:
	wxWidgets 3.0.5
	libcurl/7.71.0 OpenSSL/1.1.1g (Schannel) zlib/1.2.11 brotli/1.0.7 libidn2/2.3.0 libpsl/0.21.0 (+libidn2/2.3.0) libssh2/1.9.0 nghttp2/1.41.0

Platform: Windows 10 (build 19042), 64-bit edition, 64 bit, Little endian, wxMSW

Build Info:
	Date: Nov 18 2020 22:01:52
	wxWidgets: 3.0.5 (wchar_t,wx containers,compatible with 2.8)
	Boost: 1.73.0
	OCE: 6.9.1
	Curl: 7.71.0
	ngspice: 32
	Compiler: GCC 10.2.0 with C++ ABI 1014

Build settings:
	KICAD_SCRIPTING=ON
	KICAD_SCRIPTING_MODULES=ON
	KICAD_SCRIPTING_PYTHON3=OFF
	KICAD_SCRIPTING_WXPYTHON=ON
	KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
	KICAD_SCRIPTING_ACTION_MENU=ON
	KICAD_USE_OCE=ON
	KICAD_SPICE=ON