Convex does not support broadcasting

Convex.jl lacks support for a lot of broadcasting operations.

This causes a bunch of problems like this:
```Julia
julia> using Convex

julia> x = Variable(2)
Variable
size: (2, 1)
sign: real
vexity: affine
id: 181…059

julia> A = rand(2, 2)
2×2 Matrix{Float64}:
0.290045 0.201488
0.890725 0.771457

julia> A * x .+ 1
ERROR: MethodError: no method matching iterate(::Convex.MultiplyAtom)
Closest candidates are:
iterate(::Union{LinRange, StepRangeLen}) at range.jl:664
iterate(::Union{LinRange, StepRangeLen}, ::Int64) at range.jl:664
iterate(::T) where T<:Union{Base.KeySet{var"#s79", var"#s78"} where {var"#s79", var"#s78"<:Dict}, Base.ValueIterator{var"#s77"} where var"#s77"<:Dict} at dict.jl:693
...
Stacktrace:
[1] copyto!(dest::Vector{Any}, src::Convex.MultiplyAtom)
@ Base ./abstractarray.jl:843
[2] _collect(cont::UnitRange{Int64}, itr::Convex.MultiplyAtom, #unused#::Base.HasEltype, isz::Base.HasLength)
@ Base ./array.jl:608
[3] collect(itr::Convex.MultiplyAtom)
@ Base ./array.jl:602
[4] broadcastable(x::Convex.MultiplyAtom)
@ Base.Broadcast ./broadcast.jl:682
[5] broadcasted(::Function, ::Convex.MultiplyAtom, ::Int64)
@ Base.Broadcast ./broadcast.jl:1312
[6] top-level scope
@ REPL[4]:1
```

The purpose of this issue is to collate the currently open issues into a single thread, and to make it more discoverable for users coming to report bugs.

Replaces:
* https://github.com/jump-dev/Convex.jl/issues/410
* https://github.com/jump-dev/Convex.jl/issues/355
* https://github.com/jump-dev/Convex.jl/issues/218