Using the back button in the create external table wizard causes the create to fail

[jrowlandjones]

Issue Type: Bug

• Create a new empty database
• Step through the wizard as normal
• choose a table by selecting the checkbox
• click onto the summary screen
• click back at least twice

At this stage you should notice that the table selections have been lost. - defect #1 (no other page loses this info)

• Re-select the tables
• Click next and complete the wizard

The wizard will fail stating that the database objects already exist (which they don't) - defect #2.

Azure Data Studio version: azuredatastudio 1.7.0 (e128002, 2019-05-08T00:55:40.928Z)
OS version: Windows_NT x64 10.0.17763

System Info

Item	Value
CPUs	Intel(R) Core(TM) i7-6700 CPU @ 3.40GHz (8 x 3408)
GPU Status	2d_canvas: enabled checker_imaging: disabled_off flash_3d: enabled flash_stage3d: enabled flash_stage3d_baseline: enabled gpu_compositing: enabled multiple_raster_threads: enabled_on native_gpu_memory_buffers: disabled_software rasterization: enabled surface_synchronization: enabled_on video_decode: enabled webgl: enabled webgl2: enabled
Load (avg)	undefined
Memory (System)	63.92GB (28.08GB free)
Process Argv
Screen Reader	no
VM	0%