LB2 changes to examples

LB2 changes to examples

Since `loopback-explorer` was renamed to `loopback-component-explorer` (see https://github.com/strongloop/loopback-component-explorer/issues/123), we need to propagate this change to the example apps; specifically the same changes made in https://github.com/strongloop/loopback-example-app/pull/79:

**1.** In `package.json`, change 

` "loopback-explorer": "^2.0.0",` to  `"loopback-component-explorer": "^2.1.0",`

**2.** In `server/component-config.json`, change 
```
{
  "loopback-explorer": {
     "mountPath": "/explorer"
   }
 }
```
to 
```
{
  "loopback-component-explorer": {
     "mountPath": "/explorer"
   }
 }
```

NOTE: Many examples will not have  `server/component-config.json`, so in that case just add the file.

**3.** In `server/server.js`, change

```
if (app.get('loopback-explorer')) {
      var explorerPath = app.get('loopback-explorer').mountPath;
```
to 
```
    if (app.get('loopback-component-explorer')) {
      var explorerPath = app.get('loopback-component-explorer').mountPath;
```
 **4.** Remove `server/boot/explorer.js` since it's now done in `component-config.json`.

As long as we're doing, this we might as well also remove `server/boot/rest-api.js` and add the following line to `middleware.json`: 
```
"routes": {
    "loopback#rest": {
      "paths": ["${restApiRoot}"]
    },
...
```