Add classes to ERD elements

It's a little hard to style the current ERDs. This aims to make things
easier by associating a distinct class with each type of element within
the diagram.

I've added a common `er` class across the elements so that those that
bring their own `themeCSS` can target styles at this diagram type. This
hasn't really been done elsewhere so I'm open to suggestions; an
alternative may be to add classes to a top-level element so that we can
do e.g. `svg.mermaid.er .entityBox`.

src/diagrams/er/erRenderer.js

@@ -43,6 +43,7 @@ const drawEntities = function(svgNode, entities, graph) {
     const textId = 'entity-' + id;
     const textNode = groupNode
       .append('text')
+      .attr('class', 'er entityLabel')
       .attr('id', textId)
       .attr('x', 0)
       .attr('y', 0)
@@ -65,6 +66,7 @@ const drawEntities = function(svgNode, entities, graph) {
     // Draw the rectangle - insert it before the text so that the text is not obscured
     const rectNode = groupNode
       .insert('rect', '#' + textId)
+      .attr('class', 'er entityBox')
       .attr('fill', conf.fill)
       .attr('fill-opacity', '100%')
       .attr('stroke', conf.stroke)
@@ -148,6 +150,7 @@ const drawRelationshipFromLayout = function(svg, rel, g, insert) {
   // Insert the line at the right place
   const svgPath = svg
     .insert('path', '#' + insert)
+    .attr('class', 'er relationshipLine')
     .attr('d', lineFunction(edge.points))
     .attr('stroke', conf.stroke)
     .attr('fill', 'none');
@@ -224,6 +227,7 @@ const drawRelationshipFromLayout = function(svg, rel, g, insert) {
 
   const labelNode = svg
     .append('text')
+    .attr('class', 'er relationshipLabel')
     .attr('id', labelId)
     .attr('x', labelPoint.x)
     .attr('y', labelPoint.y)
@@ -241,6 +245,7 @@ const drawRelationshipFromLayout = function(svg, rel, g, insert) {
   // Insert the opaque rectangle before the text label
   svg
     .insert('rect', '#' + labelId)
+    .attr('class', 'er relationshipLabelBox')
     .attr('x', labelPoint.x - labelBBox.width / 2)
     .attr('y', labelPoint.y - labelBBox.height / 2)
     .attr('width', labelBBox.width)